Pretty much. As far as I've ascertained, the lack of RNG makes Lemmings runs almost entirely about execution.
For example "We All Fall Down" is pretty straightforward. Basically, the only way of optimizing the level is basically setting release to 99 as fast as possible. That pushes the last necessary Lemming out the fastest. However, to turn all those lemmings into diggers would basically require to click on every one of them while their window for clicking will basically be reduced down to a single pixel. You'd pretty much have to move the cursor slightly to the left by one pixel, and then timed each lemming movings along the cursor, turning them into diggers on their exact pixel.
Obviously, a human probably can't do this under any conditions absent the use of tools.
Regarding ESC feature, the only reason I'm really disallowing it at the moment, is because the only person outside myself to do an RTA attempt did so explicitly banning the feature, and because it isn't entirely clear to me yet which ports have some sort of ESC feature to utilize (like GB, SNES, or what have ya). I can see the pros and cons for allowing for both.
What I will likely to in the future to address this problem is to create two subcategories: One which allows ESC (or any other mechanic that can be determined to quick end the level) and one which does not.
For the purposes of your Amiga run, if you so choose to do one, I would be willing to create said subcategory if it is explicitly your preference.
As far as NeoLemmix is concerned, I've disallowed, mainly seeing how it's no way an official lemmings release. However, I don't see why NeoLemmix couldn't operate with an additional leaderboard at some point in the future, which would allow custom levels to be speedrun and get boards as well.
Another interesting thing about Lemmings, as far as TAS's are concerned, it's actually a pretty neat game TAS wise, because the puzzle pretty much remains the same. It becomes about finding the exact frames and pixels to do actions that result in the fastest possible time.
Definitely looking forward to your run; feel free to contact me regarding any questions. My graphic card has been utter trash the last couple months, so I can't really work on a 30 level run atm w/o risking crashes mid-run, but hope to address it soon.