@IchoAwesome! Icho has solved all of my contest levels with either intended or acceptable alternatives!

More specifically, R1 is an acceptable alternative, while R2 and R3 are 100% intended, with the former swapping a couple of skills around from my intended one, but it still has the essential parts of the solution in it, so no changes needed for R2 or any of my entries for that matter.
As mentioned, this is an acceptable alternative. In the intended solution, the normal lemming and climber lemming swap exits, i.e, the climber goes back to his own upper left exit, while the normal lemming goes to the upper right one. Where you used a platformer and a builder to close off the gap to the right wall, I use the swimmer/glider to platform to the left to get over land and it will always be done in such a way that the climber will be able to get over to the other side while the platformer is busy, and thus he climbs up the wall back into the miner tunnel he made at the start. For the regular lemming, after you bash to release him with the swimmer/glider, he laserers, shimmies into it, and builds at the edge of the miner tunnel. This will be enough to have him be able to jump to get over the green square block at the top. Then I have him build once he's at the edge of the upper platform, release the blocker with a walker once the staircase is able to catch him, then he turns around back to the right after hitting the green square block, then floats down to the island just above the bottom right area. Other than those differences, you managed to still need to use all the skills, and as I don't want to be picky here with completely enforcing my solution, your solution is perfectly fine in my eyes. Nice job!

What I really like about the solution I came up with is how I varied the ways to reach an exit: Simply shimmy to get over a water gap to drop down the other side of the exit safely (top left), platform up the water gap (top right), fence and swim (bottom left), and finally laserer and then jump into the resulting tunnel to get over to bypass the trap that's right in front of the exit (bottom right)
Thank you for the compliment on the visuals for my R1

In addition to this, I must say that I absolutely enjoyed making the level and I love the different parts of the solution I came up with for it! I think I even like this one more than my R1 from the previous contest even though that one I'm quite proud of too. This is also the entry that got me to finally see the light on how very useful the laserer is. Even though I have played plenty of levels featuring the skill from Lemmings Uncharted, I wasn't yet convinced of its usefulness and hence still thought that it has no place in NL. However, I guess sometimes it's enough to simply make your own levels using the skill in order to see its potential! So, I have to say that I'm now in full support of the laserer skill even though it's been in stable for about a year now.
Intended!

The climbers and jumper can be swapped and hence the player is entirely free to pick which side to use them on. My solution uses the climbers to go out the right, while I use the jumper to get past both the small block AND the trap so that you have more leeway in the bomber placement. However, the essential tricks are still there: Using the slider to laserer through the huge block to get the final two lemmings to the exit on time, platform the gap at the bottom from the right side with the slider, and bomb away the trap trigger so no one dies to it.
Have you noticed that I'm starting to incorporate the level design element of "keeping the crowd moving, you do not contain them at all" into my own creations? I know you use that a lot, and I tend to like these!

Intended!

The trick of using the platformer to contain the right entrances took a lot of backroute fixes because Armani kept finding a way to avoid using that. I was inspired by Pacifism 41 of your United pack there of using a platformer to add height to the 6 pixel wall to block the Lemmings from stepping out. I got flustered at some point, but once I calmed myself I realized that the major sources of backroutes were the builder and shimmier, and so as much as I hated to, I had to make them pickups. The only reason I avoided using them for so long is because I hate using pickups to fix backrotues, because I always have this belief that they make my level far easier than intended. However, I've been proven wrong by both you and Armani, as the both of you have told me that my R3 is still quite difficult.
I have to apologize for my solution hugely ripping off of your R3's. However, I think it's clear that my intended solution is still vastly different from your R3's intended one that I didn't 100% take your idea completely. For example, my level doesn't use sliders, but it does use shimmiers and bombers, which your R3 lacks. Also, the level incorporates a combination of "keep the crowd moving" AND contain the crowd, where the former applies completely for the left entrances, while the latter applies completely for the right entrances. Otherwise, there's so many possibilities here but if you don't take the intended route, then you'll always end up running into the problem of someone arriving too soon and splatting or getting into danger.
I think the main reasons why the level is hard is because the route for either side is not obvious at all (being able to descend down safely inside the calculator), as well as how the skills need to be used are completely non-obvious. So, I think this is another case of me underestimating the difficulties of my levels, but I'm far from being a unique case, as this is a problem that pretty much every designer has due to how it's hard for the creator to judge the difficulties of his or her own levels. A really good example is when I give feedback to Armani! Plenty of times he thought a level was easy but I tell him it's much harder than you think it is or vice versa. I also recall giving you similar feedback when I played through United or even some of your contest entries.
With this, thank you so much to Apjjm, Icho, and Armani for the positive feedback/compliments/replays for my levels!

Once again, I have Icho to thank here that I even got into level designing, as it was his encouragement to me to give it a try and without that I think I would had simply been content with just solving levels!

As for everyone else, keep the replays coming, as I always love seeing more solutions to my own levels. Not to mention others trying and attempting to get them solved

I think I will get around to playing these levels myself in the next few days or so. Just been super busy lately.
Good question. I guess that, conversationally, it really depends on how good a level I think it is. However, I think that what actually happens is that the barometer is lowered with each fix I have to make.
So, for example, let's say I make a level which I think is a 10/10 awesome level. It can be easily backrouted, so I make one fix which only changes the level slightly; at that point, I probably think it's more like 9.5/10. Then, oh dear, it can still be backrouted so now I have to really mess with the layout; now it's an 8/10. Another backroute means a third fix, this time forcing me to use pickups or some other drastic level-ruining measure; it's now probably no more than 5 or 6/10 after having had 3 fixes, and is likely to end up getting scrapped.
In another example, I make another 10/10 level (I must be on a roll!
). An easy-to-miss backroute can be just as easily fixed, so it's a 9.5/10. Another backroute means having to add a bit of steel; oh well, I can still make it look cool so it's only gone down to 9/10. Yet another backroute means a slight change to the skillset or something being moved a few pixels; I'd consider this the final fix, and the level sits pretty at 8.5/10. If it can still be backrouted, I accept that this is the case, and maybe even embrace it (make it a talisman, or even open up the level even further). This one will be kept.
The above scenarios are obviously not exact, and can happen in different ways. What I'm really illustrating is that some levels just seem to get worse and worse with more and more backroute fixes, whereas others largely remain intact with each fix. In the former example, I'll almost certainly stick to the "maximum 3 fixes" rule and scrap the level. In the latter, I might still stick to just 3 fixes and then leave it as it is, or if I do decide to keep working on it then I'll more likely go in a totally different direction with it altogether. If that happens, I almost class it as a new level rather than a new version.
Very rarely, I might provide further fixes if I reeeeally think a level is worth it. But, we're talking 1 in 100 levels as opposed to every level.
I've never thought of it this way, with each fix lowering the overall quality of the level and the way you've quantified it, but I like the way you explained it here. Interestingly enough, I've kind of felt the same way with each fix as well, although some certainly way more than others, particularly the ones where I kept fixing it up rather than stop after a certain amount of versions. At the same time, I haven't yet gotten into the realm of accepting alternatives because, well, the designs I came up with don't really allow it, mostly due to the skillsets provided. However, as mentioned I have an open-ended level from the previous contest, as well as a couple here where I'm fine with alternatives, so I apparently started allowing other solutions starting with this contest!
As you've said, everyone is different. There seems to be a positive correlation between solving skill and tenacity when it comes to level updates, i.e. the strongest solvers (Icho, Armani, yourself) tend to release multiple versions of their levels and will think nothing of getting to version 7, 8 or even higher. The mindset seems to be that they want to challenge the other players who are equally skilled, and so nothing other than the version which only allows the intended solution will do! (Of course, I could be totally wrong here!)
I think you hit it spot on here with the ones you mentioned with being adamant of enforcing the intended solution at all costs. In my case, as I mentioned the problem is that my designs haven't really allowed for any alternatives, although some of mine in this contest do! I'll certainly be more than happy to explore and get into designs with open-ended solutions in the future, but at the same time I want to make sure they're interesting puzzle-wise. I certainly have nothing against them, and I welcome them with open arms just like I do with almost any level

From what I've seen and read around here, it's easy to make either an easy or hard open-ended level, but the challenge is making them interesting too!