The consensus with #4 (sticking to downhill slopes) seems to be that it usually feels good but it's possible there are a handful of levels (although I don't think anyone has identified any?) where it would go poorly. What about a jjPlayersStickToSlopes property, set to true at the start of every level, which that handful of levels could be edited to set to false? Similar to how This Level is Untitled has a script now to make it use vanilla-like copter code?

(assuming the Speed Destruct Scenery interaction gets fixed, of course.)