Going through Danger and Psycho (and revisiting the Oddstack) turned up some more improvements (even for max saved). Most replays are only attached for the sake of completeness as the solutions require either glitches or much precision (or both). Nevertheless there is one solution, I recommend to try to find for yourself before watching the replay: Circular Wavelength (Danger 15) with only 2 climbers.

Medi 16: 1 climber, 3 builders, 1 bomber, 97%. I really didn't expect this level to have so many different solutions.
Danger 12: 9 climbers, 1 builder, no basher.
Danger 15: 2 climbers, 1 builder, 97%.
Danger 16: 2 bombers, 90%.
Danger 19: no miner and no digger. This turned out to be much harder than I expected (even after namida's warning).
Danger 28: 3 bashers. What is the solution without any builder?
Psycho 2: 2 builders, no basher. The solution with 2 builders can easily be turned into one without the basher.
Psycho 5: 20 bashers. I haven't fully analyzed this, so 20 bashers might not be final.
Psycho 11: no climber, no builder. So far I have only succeeded to save 65% without bombers...
Psycho 13: no blocker and no bomber.
Psycho 24: no bomber, 3 builders, 1 basher, no digger. The digger in the replay can be replaced by the bomber.
(any skills I did not mention above are not improved)

Here are six more replays. I am fairly sure I backrouted "Hotel in Hell" (otherwise I would be severly disappointed in this level). All other solutions feel like intended ones.
I would like to add "From the Other Side" to the list which are harder than rated. So far I have only a solution that would work if the big block would be 2 pixels thinner. But I am rather certain, that it cannot be modified into a working solution.

The database contains now an updated version of my level pack. Apart from (hopefully) removing the backroutes, I replaced level 9 with its slightly simpler version and included replays of the intended solutions.

@Akseli: Your second backroute to level 8 still works, but is much more difficult to execute (e.g. the miner has to be stopped midstroke). As removing it completely would have had other negative consequences, I will leave it at this.

@namida: I got your replays to work using the Lemmix Editor with OhNo mechanics after modifying LemmixStyles.ini. Nevertheless thanks for making the Lemmix Player. Your solutions for levels 1, 3 and 4 are intended and I would be very surprised if you get your backroute for level 2 working in the new version (though it's theoretically possible).

It seems that there are at least two other requirements for this bug:
- The builder has to be a runner as well.
- Number of Builders returned + 1 <= Available builders at the beginning of the level.
So far I have only managed to get this bug for "Circular Ruins" and "Go ahead and Help Yourself", but not for "Crimson Room", "Don't Look Back" or "Just Stop the Bleeding".
Hope this helps for reproducing the bug.

@geoo:
Level 1: intended 
Level 6: You fixed it a little bit different than I did, but that's OK.
Level 10: New solution, but similar in ingenuity and difficulty than Akseli's and mine. So I will gladly accept it. Great work !
I hope I may include your solution for level 10 in the replays I will attach to V5?

@Akseli:
Level 1: intended
Level 2: Different than my solution and accepted as an alternative way.
Level 5: The problem is not that OWW cancels steal, but that the trigger area of OWW starts slightly above the actual arrows. This leaves a gap of 3 pixels between the OWW and the steal with no restriction to terrain removal. You managed to move the basher into a position, where it checks for such things right in this gap.
Level 6: Backroute. Expect for the next version, that your first basher will stop in the middle.
Level 7: Another great new solution to this level .
Level 8: As far from the intended solution as probably possible. Though more difficult than the intended solution, I tend to remove it, as I am not a fan of solutions that rely so heavily on pixel-precise execution (and the change I plan will make it possible to beat this level in Lemmini). As for the origin of this level: I got the idea to this level while trying something incredibly stupid for qbeez's challenge to "With a twist of lemming please" (in qbeez1.dat).
I would like to include the replays of your solutions to levels 3 (for the max saved), 7 (the new one) and 10 in V5. May I do so?

@Turrican: I would be interested to include your solution to level 7 as well. OK with that?

Of course I will mention you all for finding the respective solutions.

Thanks for your feedback, geoo!
The solutions to level 2, 3 and 8 are intended. For level 6 you are almost there and I agree that FF is extremely useful (though it was one of the first levels I ever made in the old times when there was only LemEdit and Custlemm).
Level 9 deserves more detailed comments: Your solutions is rather different to mine and more complicated (though it can be simplified). But I really don't like how you turned the climber by bashing into the steel. I attached an alternative version with less builders, no steel and the "correct" RR (as it seems quite popular to change it like this, unnecessary as it is ). I would be glad, if you could tell me whether you think this version more fun?
And level 1 turns out to be much more difficult than expected.

First of all: Congrats to all of you for this great level set !

All comments below refer to the levels geoo linked to in his first post. The last time I updated my levels was late march. The numbering of the levels also refers to this list.

Levels that I found harder than the current rating:
Merge Sort (62), Derailed Level (64, far harder), Riddle me this (84, not yet solved), A fearful symmetry (94, my solution needs very precise execution), When the levee breaks (102, not yet solved), Circular Ruins (145, not yet solved though extensively tried), Get Hype (187, not yet solved though extensively tried)

Levels that I found easier than the current rating:
Adventure Playground (57), Detour (79), Flugtag (81), One way segment (83), Prelude (90, backrouted?), Abominable Snowlix (92), Switchback (101, backrouted?), Excavation (112), Hard to Port! (126, backrouted?), Three days of the Condor (132), A completely ridiculous level (133, 100% saved is easy), Logging you process (138, had this solution also for the prequel), Under the rainbow (146), Dilemma (157, backrouted?), Mice in the Pipeline (162), Watch Ye Step! (196), A Soulful bounding leap (200), Alien Invasion (207), Round Trip (218), Betcha can't save just one (223), Halfway down the stairs (230)

As already indicated, several of my solutions do not feel like intended ones. The most prominent examples are attached.

There are also some levels that deserve special comments. These comments contain mostly criticisms, because the list of levels that deserve "Great level" is just too long...
- Roundabout (39) @Möbius: Can you please check whether this level is actually solvable. There is a pesky gap at the top between the boxes and the steel to the right, which isn't there in the Lemmini level.
- Migration (67) and Last Mile (75): I don't understand the time limit. Every solution I can think of fits within the time limits (at least after two or three tries)
- Can't reach it - don't need it (70): Wrong title - the lower exit can be reached (which isn't even that hard btw).
- Cold irons bound (78): May I suggest a sequel without any floater (but everything else the same)?
- Too much stepping stones (105): This level gets much easier with basher- and miner-stairs (and even then it is non-trivial to pull off a solution). But as far as I know, there are no earlier levels showing off this trick. So for inexperienced players, this level is MUCH harder.
- Trivial Matters (107), Subversive Activities (152) and Elixir (197): Trivial matters and Elixir are (up to a slightly moved exit) the very same levels (unless I missed something). Subversive Activities uses the same general approach (but has some twists in its execution). Doesn't one of these levels (preferrable Subversive Activities) suffice?
- The same comment, but to a lesser degree, concerns: Derailed levels (64 + 189), Logging your Progress (54 + 138), Backslash (98 + 168)
- Impetus (113): It's fairly easy to save 100% using 2 climbers, 2 miners and 1 jumper. Intended?
- Corridor of Death (179): Is hiding the trap really necessary?

And finally one question: Is there a list with all intended/officially accepted solutions (except the 49 pages of this thread)? I would like to compare my solutions to the ones already known, before posting them here and stealing your time.