Tricky 4 is definitely possible...
Also I completed Mayhem 18, 19 and 28. (posted a replay of 19 in the other thread because I'm quite proud of working out the tricky bomber timing
)
And Tricky 24 (Tightrope City) is possible too. It should be doable at 100% with the same release-the-blocker-and-turn-him-around trick that you used for Fun 9, but I'm not very good at pulling that off. (basically, you do a two blocker trap with one directly underneath the entrance, and then once you've used 1 builder to go over the gap, you can release the blocker under the entrance, and then you can probably release the other one too or something, I dunno, it's easy to solve, anyway!)
And here's Tricky 25, which is non-trivial and therefore quite fun. I saved 70/80, which is a lot more than the "normal" solution going forwards, but I don't think you can save 100%...
Taxing 13 is also possible (replay on request) – I ended up saving the minimum 98%, though, because I couldn't be bothered working out how to release my blocker (I had run out of bashers, so I tried with a miner which didn't work, but maybe if I put the blocker higher up on a stalk it might that way)
I suspect Taxing 14 is probably possible, given that it's not a particularly hard builder level to begin with. I'm not sure I can be bothered trying it, though.
Taxing 16 is also possible (again, replay on request) – I did it for 97% and finished with literally no time to spare, though, because building up to the exit takes a bit of time and there is only 3 minutes on the clock (the original level had a bit of this but not to the same degree). That said, I could have comfortably exploded the blocker a wee bit before I actually did, so it's my fault really for making the timing so tight.
Taxing 22 is possible, although I don't know about 100%.
Taxing 23 is possible – same solution as the tricky level.
Taxing 24 is possible and not hugely difficult – I think there are a couple of possible ways of doing it.
Taxing 26 is possible. It might be mildly more difficult to get to the top exit, though. I haven't tried yet.
Taxing 27 is possible (just have to bomb down).
Taxing 28 is possible. I haven't ascertained whether it's possible on 100%, but I had a few skills left over, so it may well be (unlike the forwards level of course!)
Taxing 29 is possible. It's not as trivial as some of the others, though, as you have to build up to the mound on the left using the 5 builders, and then you need to climb up the last little bit in order to block and release. I've managed lose 2, and there's a replay if you want.
Taxing 30 is definitely impossible with the current skill set – I think all the We all fall downs will have the same extra skillset needed, but I'm not sure what that might be.
Mayhem 1 is possible. I dunno about 100% though, I did it with blockers for lose 2. There's an awkward bit when you're going down where you need a floater, and you could probably use an extra blocker and lose 1 more to make it less tedious, but I (having found Lemmix's skip-ten-seconds function, which makes building much less tedious!) built over to the exit and back again to make the drop safe.
Mayhem 2 is also possible, and again, the fact that the level has floaters comes in handy.
Mayhem 3 is definitely impossible with the current skillset. Given the time limit, I'm not sure if it will be possible with any skillset, to be honest. (With the Fun 24 skillset, I had to build to get up to the exit, which took quite a long time.)
Mayhem 4 is impossible, but can be made possible with 3 builders and 2 blockers. (same way I did Fun 25)
I'm pretty sure Mayhem 5 is impossible with only 5 builders, but feel free to prove me wrong here!
Mayhem 6 is possible, and fairly easy (just build through the gap).
Mayhem 7 seems to be impossible. I think there is a possible solution if you add a digger and a miner (I trapped the lemmings to the right of the entrance by digging down – this way a climber can get up, and later in the level you can dig+mine down to release the lemmings). However, there may be something I'm overlooking here. I'm not sure I know the "best" solution to the forwards level.
Mayhem 8 is possible, and not too hard.
Mayhem 9 is possible too. Not sure about 100%, I used the blocker. Should be possible though, because you can probably make a digger trap and there are plenty of skills left over.
Mayhem 10 is possible. It's quite fun as a multitasking level, too, and getting down from the Pillars is quite interesting, so it's a bit harder than the original overall. I managed a lose-4 solution – although I'm sure that's not the maximum, I'm not sure about 100% because I needed a blocker for the right entrance. (Also, what the hell is with Lemmix's entrance order? That confused me for a minute there when the lemming didn't come out of the correct trapdoor!)
Mayhem 11 is impossible, as we know.
Mayhem 12 looks impossible to me, or at least too difficult to attempt at 1 in the morning. However, I know that Clam can glitch through the wall, so that might be our best option here, if it's indeed possible.
I don't think Mayhem 13 is possible from a cursory glance over it.
Mayhem 14 is fairly symmetrical, so should be as possible from the right as it is from the left. I haven't attempted it, however.
Mayhem 15 is possible.
Mayhem 16... not even going to attempt it to know it's impossible.
Mayhem 17, again, is very symmetrical, and should be as possible from the right as from the left.
Mayhem 18 & 19 are possible.
Mayhem 20 is impossible; as impossible as Fun 4 was.
Mayhem 21 seems to be impossible from a cursory glance, but I think it might be possible with one extra basher.
And as I mentioned, it's about 1 in the morning here now, so I hope this has been of assistance.
I'll let you guys finish the levels off and confirm the ones I'm not sure about.