-Is it possible to create triggers to force units to automatically destack if they stack, with say an overlord, larva etc.
- how do you edit unit supplies?
-can/how do you edit spell duration, effects, cost. etc?
None.
NudeRaider -- blessed innocent soul knows nothing of the strife we had before EUDs were discovered 
DarkenedFantasies -- Probably just didn't care. For example, at some point before release, they've updated the graphics of some of the Protoss buildings (Forge, CyberCore, Citadel, Observatory, Arbiter Tribunal), but instead of properly re-rendering them with edited 3D models, they did crappy copy-paste jobs on the rendered graphics.
Symmetry