21: could have been an alternative solution, but Stoner staircases are absolutely not the thing required here. I've added a really ugly long vertical steel wall to hopefully limit the space too much for a Stoner staircase to work. Also, I continued it all the way up to the ceiling, just to be on the safe side.
Since you only have 5 Climbers on this level, I don't see a way this steel wall could be exploitet for backroutes by itself.
22: Intended (talisman).
23: This is the same alternative solution somebody else (I believe Gronkling?) has already found, so this is fine.
24: Small variation on the intended solution (again, similar to Gronkling's): You bashed to connect the two crowds at the very bottom, instead of bashing right behind the Builder to cut off his staircase behind them (Pieuw's signature trick), which would have been the intended solution.
25: Intended. The Climber is optional.
26: Close to intended. You could have spared that Blocker if you had made use of the splitter to turn the Miner around instead. However, in order to get to the splitter from the left to flip it once, you would have had to expend a Bomber, so it still would have resulted in a lemming loss.
27: Open-ended, fine solution.
28: Open-ended, fine alternative solution; the way you got over the exit with a Stacker and a double Miner was a little different from my approach: I just mined away the exit trigger first with a single Miner, then later on re-connected with it using a Platformer.
29: This is the first backroute that uses every skill. Specifically, the backrouty part is the Digger on the left, that was close enough to the edge of the water that a Glider could simply get over there. I maybe could have allowed this as an alternative solution earlier, but I've already gone so far out of my way to enforce the intended solution (among others, by making the Shimmier a pickup skill, too) that I might just as well go all the way. Very sorry to have to break this solution! But the intended one is truly one of a kind, so as long as I do see clear ways of enforcing it, I simply have to take it.
In this case, that meant adding a little more steel so that now, the earliest place you can dig is still too far away from the edge of the pool to safely glide over it.
30: Yeah, this one pretty much has to remain open-ended, because the only way to enforce the intended solution without completely ruining the levels appearance through a bunch of rectangular steel blocks would be if we still had manual steel...
One-way arrows wouldn't cut it here, because basically all alternative paths I've seen anybody take on this level invole Bomber shortcuts.