Re 1, my proposal was to no longer disallow assignment to jumpers in the first place. ie. if you click a jumper with Builder selected, he immediately starts building just like a walker would.
This is a very bad idea due to the reasons ccexplore (no improvement for downwards slopes) and IchoTolot (more backroutes) mentioned. Moreover allowing to start builders, platformers and especially stackers mid-jump has the potential for lots of incredibly annoying tricks/glitches (I can provide example levels if needed). I would rather keep the old handling than allowing to start skills at some intermediate height.
Perhaps I should have given more background on the reason, why I made this suggestion in the first place:
When already pausing, the frame advance options are already good enough so that further improvements are not really needed. But when I play an easy level where it doesn't really matter where I assign a basher/builder/whatever, I just want to assign the skill. Currently I have to pause the game for this (or hope to be lucky), which I find dissatisfactory. Even if there is no big slope, but only some bumpy terrain (grass in the Dirt style or on rocks in the Rock style) it sometimes happens that a skill doesn't get assigned, due to unlucky timing. When this happens during fast forward or when I press 10sec-skip directly afterwards (without double-checking the assignment), it is very annoying.
So I definitely don't want to add new positions where non-permanent skills can be started, but only ensure that those unlucky skill assignments will still be executed at the earliest moment, when such assignments are regularly allowed.
Assignment to fallers is much more game-breaking than assignment to jumpers...
That is the reason why I suggested to remember the skill assignment for a few frames until the lemming reaches again solid ground, where he can regularly start working.
2. Should the one-assignment-per-frame limitation be kept?
I would put removing this limitation as [LOW-PRIORITY]. While it would be nice to remove the problems with colliding assignments from player and replay, it is not really game-breaking. I cannot think of any (non-artificial) level, where changes here create new backroutes.