1. Trample will not be so strong (instant kill) like is in 1.2.
2. There will be always correct hitbox.
3. It is possible, but it need testing, I am not sure if it will make better gameplay or not. Also better collision detection will give advantage to aliens, because you can hit them only by shooting at body, not at collision box.
5. Yes.
6. Yes - scars that will be more visible with more damaged player.
7. Do you reading our gameplan?
We have planned "gravity mod", but it is very complicated thing, and we are not sure, if it is even possible to make it real. For singleplayer game it is not problem, but multiplayer is way harder. We will work on it when everything else will be done - maybe after game release.
8. Building on top of each other - yes (server option).