I'll try to answer these as best I can.
Q: -Can I influance gravity? For excample remove grafity in a map but make the bipeds and vehicles fall down to the nearest surface. So kind of having players walk on walls and vehicles drive on them.
A: Not sure what you mean by this. By using a constant script, you can change the gravity of certain areas of a map. Also, you can adjust a biped to be able to "wall walk".
Q: -I haven't looked into this at all yet but I just ask it now: Can you still script things like in halo ce?
A: Scripting is still the same as it was in CE, just different syntaxes and strings...obviously.
Q: -Can I still switch bsp or skies?
A: Need more info. If you are referring to changing skies to a custom one, then yes. Switching bsp's is still unexplored...mostly.
Q: Can I make players in multiplaer on my map have flashlights?
Q: -In the last level of singleplayer (I forgot the name at the moment) you fly whit a banshee to protect a scarab. If you try to land on top of the scarab you will notice that it is like the scarab has inviseble magnets to summon the banshee in place. Can I make those in my own maps aswell?
A: The level is called deltacontrolroom.map There are no invisible forces at work directing the banshee to the scarab.
Q: -Can I make distructeble objects whit working collision? If not maybe as part of the bsp?
A: Yes and no. The only bsp destructable object you can make is glass. You can make custom destructable items, with collision (coll) but the one thing we are missing, is a way to make perfect (phmo), which is also known as physics models.
Q: -Can I make tv screens that actually work? Normally I wouldn't even ask but when you look at the sniper rifle you notice it has this little screen working. Can that be used in any other way?
A: Personally I dont know anyone who has ever looked into this.
Q: -Can I make a surface switch texture/bitmap on command (like scripted).
A: No. Bitmaps or shaders are not script controlled. They are setup to switch or alter by the tag that calls on them. For example the warthog. The vehicle tags themselves call for a differetn shader to be shown for destructed parts of the vehicle. Same would apply for the rest of the game.
Q: I'm mostly intrested in doing these things in a way the maps still work as normal in multiplayer even the way of downloading the map. So if it can only be done using changes to halo please mention that.
A: Unfortunatly there is no perfect map editor for H2V. Any thing you use will end up corrupting the original map file. The only way to make these changes would be to rip the H2X bsp's, and make a new map from scratch.
I hope these answers help you a little. If you want to stay in the know about H2V progress and cool maps, I suggest you go to the H2MT's (Halo 2 Map team's) home site called... http://www.monstrmoose.com