everything possible, but some things are hard to script and a bunch of work.
1) only scripted, so to change in a specific spot and limited gun numbers. you have to use variables to check for character and character-settings as health, carried weapons etc. you have to build your whole game and levels around this feature, but should be possible in the final.
2) use rpg-mod (not for beginners) or a system i think conjured entertainment, rolfy or bugsy did, wich is usable in original versions without a mod. its anywhere here in the forum using a mainlevel (elevator or something) from where you can visit different levels.
3) use rpg-mod.
4) part of 1) because in an fps you ARE the gun.^^
5) was around somewhere in the modelarea or tgc-store, knightgloves firing firbolts and lightflases.
many things are possible, even on a smaller scale than in progames because of limitations or performance. but the most things you asking for are definitely not beginners stuff.