Level 2 "Gorge":  a very easy level.
Level 3 "Fiery Cavern": a straightforward level, with one advance lemming forging a path for both groups, while one group is held back, before they join up.
Level 4 "Path of Fire Twins": shows the advantages and disadvantages of Sliders. Needed a bit of tweaking to rescue all the Blockers, but not too difficult.
Level 5 "Frozen Pass": unless my solution is a backroute, some of the skills provided are red herrings. You need to know how to create an upwards path with Bashers, Miners and Builders.
Level 6 "Room of Shredder Traps": a step up in difficulty. 1 advance lemming needs to create a path for the others. That part is straightforward, but releasing a second advance one, some time after the first, is a challenge; the Slider plays a vital role. The solution needs perfect placement, both early on and near the end.
Level 7 "Swastika": Equally difficult. From the bottom trapdoors, the first lemming does the advance work; from the top trapdoor, the last does the key work. I had quite a few false starts with different ways of delaying the second lemming at the bottom left – most of which left problems elsewhere. One of my solutions uses the Stacker to delay; the other uses the Stacker in the obvious place, with a Miner doing the delaying.
Level 8 "SnowLem Land": a breather level. Needs the Blocker to turn a Builder, and careful attention to the topography to rescue the non-Climber from the right-hand group.   
Level 9 "Hard as Steel": an easier puzzle, but still needs a bit of figuring out to ensure that the 1 of 5 lemmings that cannot be made a Climber is delayed until a rescue platform is in place. In fact, only 1 Climber and 1 Miner are needed.
Level 10 "This Puzzle is Crystal Clear": I hate spider's webs, but even so it was not too difficult to save 99. However, care is needed to avoid the Hang Glider overshooting the exit. First I found a complicated solution, with the advance lemming bashing back to free the group, where I had to: time things so that the first lemming liberated by the advance one is the 100th spawner, which is only slightly ahead of the 1st (which follows close behind after looping round); increase the distance between 100th and 1st so that the 100th can be sufficiently advanced in its mining so that the early followers do not simply walk past the mineshaft; and save the Hang Glider, while still allowing the later lemmings to avoid the trap. Precise timing is the key to the first challenge. The spare Climber/Floater can help with the second. For the third challenge, a sneaky bit of pretend mining did the trick. But then I realised there is a simpler solution, which leaves 3 spare skills.
Level 11 "Center Point": Timing is very tight here. The first Stoner has to go in the middle. The second Climber is used only after the middle is reached.

With only 1 of 100 lemmings to be saved, and only 2 Bombers to get through 3 pit floors, a Nuke solution is clearly needed. So the final 2 lemmings have to reach the exit area before they explode, but they cannot access it until an earlier spawner has exploded. If I understand the game mechanics correctly, the time interval between the first and last explosions depends on how many lemmings there are between them, still alive at the point of Nuking. This creates a problem: if there are too many exploding lemmings, the ground at the bottom is blown away. So how to avoid this while still keeping most of the lemmings in play? I used a Builder and Blocker to corral the group to the right of the middle pit, while allowing the first spawner to escape. Timings and release rates have to be handled carefully. My final 2 lemmings were not tight together when approaching the pre-exit trap, but fortunately the trap is relatively generous.

Level 13 "Failed Hibernation": the traps are so rapid-fire that, for 1 of 5 Climbers to pass the 4 traps, the 5 need to be so tightly bunched that they appear on screen as a single lemming. Even this may not be enough, with the chameleon traps firing every 5 frames – far faster than a pair of Climbers can get through these traps. This made me think that bunching up the lemmings between a pit wall and a Stoner would not work. Eventually I realised that it's OK for each chameleon to gobble 2 lemmings; and that making the gap between a Stoner and a pit wall as narrow as possible is not a good idea. Needs perfect placement and timing, but a nice puzzle.
Level 14 "Salvage Operation": another challenging level. One key element for my solution is making sure that, when the group are freed, the first lemming to advance is a singleton, not one of the pairs.
Level 15 "Arrows": a breather level. Lemmings from the middle group free the left and right groups. I got the gold talisman on my second attempt.
Level 17 "Castle Conqueror": a good challenge, a large part of which is how to delay most lemmings while facilitating the necessary advance bridging. The 3 Shimmiers are used in 3 different ways, one of them a variant of a trick learnt in the first level.
Level 18 "Hall of Hope": not difficult to meet the save requirement and time limit, but my first attempt to do better ran into problems with running out of Platformers and using more Bombers than I wanted to. What was needed was an economical way of releasing at least 3 advance lemmings: the generous Climber allowance suggested a setup that allowed only Climbers to advance. Eventually I was able to save 74 lemmings (the requirement is 70) in a reasonable time.
Level 19 "Hydroelectric Powerplant": I learnt a new trick here, making a stack ascendable by using a Fencer, some distance away, to make a notch in the middle. Not too difficult if the skills are used frugally, and another generous time limit.
Level 20 "Watch Left And Right": an easy level to finish the rank, with lemmings from the right-hand trapdoor doing most of the work.

Correct solutions
The Bad Beginning - Of course
We Are Lem-Bob-Omb!
Lemstones - You used the intended side.
Cloud-Covered Stalactites - Your solution is the same as JawaJuice's. Rewatching my replay, I noticed that I only use 5 bashers so I may tweak the talisman to require a more optimized (and elegant) route
It's a Kind Lembourhood
Minimal Design - Your second replay is correct! You can make it a bit easier by setting the three blockers right away at the beginning, as setting a blocker among packed lemmings is sometimes a nightmare
The Italian Job - Well done! There are ways to ensure lemmings won't splat, execution doesn't have to rely on luck here. You can take a look a JawaJuice's replay for a good example of this
Pagoda St.
Voltaic Mocaics - Your 100% solution was fun to watch
What Happened in Sovogda? - Different from my solution but still valid.
Iron Industry - Here I only wanted to make a fun puzzle, divided in several little steps. The pickups making the order obvious is not an issue to me

Backroutes
Emerald Cave - Clever, but not intended. Won't be too hard to fix I guess
Better Run for Shelter - Right side is correct, left side is not. Fixin JawaJuice's backroute should also fix this one.
The Strange Relics of Lemnos - Your talisman solution uses the whole layout so it's kind of right but this basher/blocker trick makes it messier than the intended route. It even makes it harder as the intended solution doesn't require such timing or precision. I may remove blockers from this level as they're not even useful for the vanilla solution. Or I could just add a requirement to the talisman.
Standard Test Chamber - Very nifty solution! Not intended, but I like it. Like JawaJuice's it required way more precision than the intended solution though.
Clumps - Yes. I will need to rethink about this level for sure! Even if your solution it really solid, it's very far from what I had in mind for this level.

Alternative solutions
Training Day - Interesting! I intended for all the lemmings to get to the left side exit. But you still used the basher correctly so I'd say your solution is valid too.

Glitched?
Lemming Falls - Not sure what's happening here but your replay only saves the worker, the other lemmings all splat. I also tried the replay from your previous attachment with the same result. What floating piece of grass are you referring to?

Not solved
Ancient Crash Site - I won't say much because I don't want to give unwanted spoilers, but a good part of your solution is correct.
Spring Break - Well done nevertheless. This ascending basher stairway is a work of art


And to answer a few of your remarks:

Lemming Falls

QuoteI love the water fall in this level, tricky design 

I was worried about the vertical water looking weird but it kind of works so it's nice


Standard Test Chamber

QuoteOne remark though : two heart cubes ? There is only one of them in Portal 1 for sure, and tons of them in Portal 2. Breaking the unicity or the extreme multiplicity of those cubes here seems like a weird idea.
Also a suggestion : definitely put a tiny "The cake is a lie" or "The lem is a lie" text in some corner of the level !

Ah, as usual I got carried away with decorating this level and put another cube to the right side. Me and my symmetry obsession Removing the second cube would make sense. I thought about adding other Portal references but 1/ it's haaaaard and 2/ I didn't want to make the level too on the nose. Buuuuut anything may still change of course

QuoteHow does it compare to JawaJuice solution ?

I'd say it's similar in the general idea and trick used, but yours is cleaner, if I can say it that way. Not to dispraise their solution, of course!


What Happened in Sovogda?

QuoteWhy not set the depart screen to this moon and starting area, as you did in The Italian Job starting zoomed on the helicopter ?

I tried to set the start view as high as possible, keeping the floor visible. Making only the moon visible would be annoying as the played would have to scroll to reach the trapdoor. Speaking of this, I'm not sure what I did is optimal for all screen sizes. I only tested this on my laptop so it may vary for other users About vertical levels, I set their width so it would exactly fit on my screen without having to scroll to the sides but here again it may not work for all screen sizes.
Also, it's not a moon and the whole level is just a big reference.

Emerald Cave This one was hard to spot for me : after the builders we realize that we have 2 builders left to contain the crowd somewhere. I could do it in a precise spot at the right of the level, intended I guess ?

Lemming Falls I love the water fall in this level, tricky design 
I used the floating piece of grass to avoid the splat for the crowd, not sure if intended.

The Strange Relics of Lemnos Still dont know how to do it without cancelling the upper left basher in some mysterious way.

Minimal Design I guess I have the save 15 solution you mentionned earlier. Gotta make splatforms in an optimal way : one bridge to both avoid splat and go over the blocker previously set for turn around.
It did not take me long to figure it out because, luckily, I just played the Mobilems pack in which the level If a Lemming Falls uses a very similar technique. This level had me stuck for a week!

The Italian Job Execution was pain here : mine too early and lemmings may splat if they have not joined the crowd yet ; mine too late and you get a bunch of close lemmings some of which may fall from one pixel too high and splat.
There must be something I overlooked here to make the miner easier to set.

Standard Test Chamber Wow ! A tribute to Portal in a Lemmings level, great idea. The two best puzzle games ever in one, brilliant.

One remark though : two heart cubes ? There is only one of them in Portal 1 for sure, and tons of them in Portal 2. Breaking the unicity or the extreme multiplicity of those cubes here seems like a weird idea.
Also a suggestion : definitely put a tiny "The cake is a lie" or "The lem is a lie" text in some corner of the level !

Anyway, great level. For the resolution I came to the conclusion that one needs to first get to the middle part to push the button, then get to the upper part with the crowd to the exit. So in the meantime I had to fill the hole leading to the middle part. Was that the intended way ?
How does it compare to JawaJuice solution ?

Voltaic Mosaics Not that hard even with a single miner. I must apologize that my solutions here are a complete mess as I was improvising most of the time.

What Happened in Sovogda Love the design and the red moon. Why not set the depart screen to this moon and starting area, as you did in The Italian Job starting zoomed on the helicopter ?

Clumps I also have a save all and no destructive solution. Walkers are that strong.

Iron Industry Very nice level. Regular clear does not need any pickups at all, and talisman needs all of them. As often with pickups, I cannot decide if they make things harder or easier actually. I mean looking at the level you can quickly see in which order to go and pick them, and this kind of gives away the solution. Though if you were to give the skills readily available, then the level would fall apart. So yes, pickups are double edged.

1) Open the replay in a text editor.
2) Find any line which matches the following:
   n, 2, FLAPPER, lem   (where "n" is the frame number and "lem" is the lemming index)
3) If the replay was made with the Timed Bomber option enabled, type ", true" at the end of the line. Otherwise, type ", false". Examples:
   162, 2, FLAPPER, 0, false   (an untimed bomber assigned to lemming 0 at frame 162)
   394, 2, FLAPPER, 5, true   (a timed bomber assigned to lemming 5 at frame 394)

Done!