Mutilation 1 to 5 - All intended. Re: Mutilation 4 "Jump Scene", the extra basher is just there as a decoy.
Mutilation 6 "Lemming Rotation Club" - A bit different to how I did it, but definitely acceptable.
Mutilation 7 "Rockroll" - A known alternate solution; I nearly treated this as a backroute when I first saw it but then realised the most important trick is still used, just in a different place.
Mutilation 8 to 9 - Both intended.
Mutilation 10 "Twisted Track" - Not sure how I feel about this one. Technically, it has the important elements of the intended solution, but it just feels too simple, I don't know.
Mutilation 11 to 13 - All intended.
Mutilation 14 "Started From The Bottom" - Hm, this one I'm not too sure about. But this level has also been more of a pain than I feel it's worth backroute-wise, it isn't a particularly great level even without backroutes...
Mutilation 15 "Snow Place Like Home" - Intended solution.
Decimation 1 "Mastery of Malice" - Very borderline, but I think it's acceptable.
Decimation 2 "The Winning Edge" - Okay, this one is a definite backroute.
Decimation 3 "Lemming Semantics" - Hm, this level already got moved back quite a bit from its original position - to be fair though, it's also a lot easier than its original incarnation (which had to be heavily changed due to backroutes). Intended solution, by the way.
Decimation 4 "Oh Blast..." - Intended solution. I actually intended to use this variant of this trick originally in the Lemmings Plus IV level "Comradical!", but getting the timing to work well enough to enforce the trick would've required everything else to be pretty much pixel-perfect, so that was scrapped in the end - the stoner in the skillset is a remnant of this.
Decimation 5 "Levelly McLevelface" - Intended solution. Though I wonder why everyone bombs in that spot - to me it feels much tidier to bomb on the flat bit just slightly before where you bombed. :/ (Of course, this makes no practical difference to the solution, so is not in any way a backroute.)
Decimation 6 "Baby Steps" - Intended solution. I have to wonder, with how quickly you say you solved it, if something about this level is especially easy for you (eg. have you used similar tricks in your levels a lot?) I wouldn't think this is a very easy level overall, I actually consider it one of the harder Decimation levels.
Decimation 7 "Expedition" - There's no intended solution here, it's an "anything that works is valid" level (as you probably guessed), but nice solution!
Decimation 8 "Mission: Difficult" - Well, that makes four different known solutions for this level now - and I feel this one is acceptable too.
Decimation 9 "Return to Gemland" - Intended solution.
Decimation 10 "Across The Sky" - So, this was originally a late-Obliteration level (the original solution still works), but was removed at a very late stage due to backroutes. It was brought back at an even later stage after one tester pointed out "even with the backroutes, it's still a fairly hard level, just not late-Obliteration hard". So there isn't really a single intended solution for this one. But I also feel this solution is maybe
too easy... hm...
Decimation 12 "A Lot Of Lemming Going On Here" - This one was one of the biggest offenders for backroutes during testing, so I can't say I'm super surprised another one has popped up.
Decimation 13 "Offer Void In Lemmingland" - Intended solution.
Decimation 14 "Bricks Are Heavy" - Intended solution.
Regarding Decimation 11 and 15, without an attempt replay I can't really comment on what (if anything) can be done to either make your solutions work, or more clearly block them, as appropriate.