Indeed, I basically had the solution, except for making the first of the left-hand lemmings a climber rather than the second, then having him explode on falling as he fails to climb! A particularly fiddly little execution, but not atypical for LemFan so probably should have expected something like that

The first Lemming from the the left entrance should be the climber AND the one you bomb instead, when he has fallen back down, therefore allowing the second Lemming from that entrance through instead of falling to the bottom like the others

I can almost do it, by making the second lemming from the right-hand crowd build to the left, then making the second lemming of the left-hand crowd a climber while the first one bombs away the OWU floor, but because the builder goes through the thin wall, that turns the climber around.

Taxing 3 I, Me, Mine, Miner : One of my favorites. I'm usually not that good with miners, and this level taught me again how  to use them as delays, really nice.

Taxing 4 Symmetrical Structure System : Backroute I think ? I save 79/76 and spare many skill : just build a giant splaform by escaping a athlete, and free the crowd by a nice bomber.

Taxing 9 Slow Esthetics : This was hard to me ! Using the floaters at different places to allow builders to set up 2 bricks instead of one before the next lemming come was the key. Frame precise but fair I'd say.

Taxing 10 Pillar Confusion : those pillars are indeed confusing, great design ! Backroute again I think as I save 24/22 and have leftover skills. A single worker was enough, the long digger at the beginning helped a lot.

Taxing 14 Bubble Trouble : Mean time limit ! I had to use not less than 5 workers at the same time, so this level must please kaywhyn ! I end up with many skills left again and save 20/15 at the very last second, so this may be unintended.

Mayhem 8 The Law of the Jungle : I save 79/76 by going straight to the exit. The crowd is freed by using the pixels removed above a miner's head when it starts.

Mayhem 12 Shorty Deep Inside : I spare a miner and a floater in this very limited skillset. This is possible by catching all those not-so-close climbers with a builder, but it is frame precise and needs many tweaking on all required lemmings.

Mayhem 14 Lemmings Fever : I love this 1 minute level ! There are so many solution without the timer, but I guess only the most efficient of them work in under a minute. I loved the use of the bomber.

Mayhem 15 It's All About The Style : Excellent level ! The use of the diggers to free the blockers at the end is very ingenious, took me a long time to find.

Mayhem 16 The Scaffold : Is that a backroute ? I save so many skills that I think it is. Hardest part was to figure out the left blocker.

Mayhem 18 The Enigma : the miner extension was a must to spot. My way of sneaking under the OWW in the middle looks unintended. It saves some skill and does not use the rightmost part of the level at all.

Mayhem 20 Kreuzung At The Mountain : I love this marathon level, mainly gotta get the left lemmings to the right exit, and conversely, with nearly unlimited builders. I played it several times to try to optimize and could save 99/95. Could not make 100% save though but I'd say it is doable.

Mayhem 21 Enter At Your Own Risk : I agree with Icho and others here : this level is great. Nuke solution did not occur to me that fast because the save requirement of 72/80 is pretty high. So one has to optimize correctly to reach it, but execution is not hard actually once you know how.
This reminds of a certain One 5 level in the Save One pack, though I certainly created it before playing this Mayhem 20.

One extra Lem saved, certainly by sparing a bridge over what's intended to stop the miner before it falls to the bottom.
To achieve this you have to place the bomber, miner, and 2 builders in pixel-perfect places. See solution if interested.