2. Not intended. I need to think more about this one before deciding whether to declare it a backroute or not, though I am leaning towards yes (it is one).
3. See above post. I know you hate spoilers so I tried to keep it as vague as possible while still explaining why you probably won't hate this level as much as you're thinking now.
4. Not a backroute. Just slightly different timings than the intended solution.
5. That would require shortening the gap, no? Originally the gap only required two builders to pass, but this lead to backroutes as the second lemming didn't need to be delayed nearly as long. I didn't go as far as testing exactly what the cutoff gap size is to avoid them - just that the size of the gap now is big enough.
6. Backroute.
7. There's a lot of approaches that
look like they might lead somewhere, but this is (as far as I know) the only approach that actually
does work.

9. Backroute.
10. Another borderline case, though this one I'm leaning a little bit more towards the "acceptable" side.
11. RR fiddling is not nessecary. I don't know if it remains true in the final version, but at least at some point during the designing / testing of this level, the choice appeared to be either tight digger placement, or backroutes. (It's worth noting the early version had one extra builder, which I later found was unnessecary.)
12. Yeah, backroute. And no matter how good this solution may seem, it's getting fixed, because the intended solution to this level is even
more impressive.
14. Damn, I thought I finally got this level solid, but appears not (this is one of the levels that was
extremely backroute-prone during testing). This is gonna be a hard one to fix too...
15. Another borderline case.
18. I know you don't like hints so I won't give any. I'll just mention that the only person who's solved this level so far did mention that, once all the backroutes were fixed, they think this is the pack's hardest level.
19. Interesting. This solution is somewhat of a hybrid between mine and IchoTolot's, perhaps a bit closer to mine than his.
20. Major backroute, though I'm sure you already guessed that.