Amazing that you got exactly 25 btw - how exactly does this trick work?
Internally, NeoLemmix keeps a list of all lemmings in the level. At the start of the level, all preplaced lemmings are added to this list, in the order they come in the level's data. Then, each time a lemming is spawned - be it from an entrance or via a cloner - this new lemming is added to the end of the list. Note that a lemming is
not removed from this list when it exits / dies / etc, it just gets marked as "no longer active", though this distinction does not matter for the purpose of this trick (it can matter in other contexts, eg. manual editing of replay data).
When the nuke is used, a 5 second countdown is immediately assigned to the first lemming in this list that is still alive and active (any that are no-longer-active are skipped). Then on the next frame, the next lemming on the list gets one, and so on, until every lemming has gotten one. (Prior to V12.7, zombies were skipped just like no-longer-active lemmings are. As of V12.7, zombies are no longer skipped; all lemmings that are still active will receive a countdown, one per frame, even zombies and neutrals.) As soon as the nuke is activated, hatches also stop releasing any further lemmings.
The trick in my solution above is that the first lemming is allowed to go into the crowd, but the next 34 are contained off to the left. The final 15 are then allowed into the crowd again, and the crowd gets gathered above the exit. Nuke is activated. The first lemming begins his countdown. The next 34 frames are spent assigning countdowns to the lemmings that were seperated from the crowd; by the time countdowns get back to lemmings in the crowd, it provides them with enough time to exit. Thus, 15 of the 16 lemmings in the crowd exit. The cloners are just used to bring that 15 up to 25, so the save requirement is met. (In a more elaborate setup, like the hypothetical "extreme cases" for Fun 21 and 23, you might need cloners to actually get lemmings to explode at the right time, or to add delays between countdowns starting. In this specific case, they're only needed to meet the save requirement.)
It's no coincidence that I got exactly 25 - it's because I allowed 16 lemmings (one of whom is lost due to exploding) to go to the right, then add 10 cloners. I suspect it's possible to allow a few more than 16 (though not a full 26, hence why
some cloners are needed - if you try this, some lemmings will explode before exiting, remove the ground below the exit, and thus leave others unable to exit either; it's critical to seperate some lemmings off
but keep them alive) to go to the right; which could either result in using fewer than 10 cloners, or saving more than 25 lemmings. I'll leave optimizing the solution to someone else; I just wanted to prove there
is a solution.