Oh, even on flat-screen tv's the splitscreen doesn't use the whole screen dude. It was like it on RE5 aswell, no matter what set up you have, when you do splitscreen, it's 2 slightly smaller screens with some black around the edges. Never could figure out why it's like that but holy **** is it annoying.
Once you've completed a campaign and found all the guns, you can start any campaign from any chapter beginning with all the guns, you have to find them the first time around though. Just click 'chapter select' and choose whichever chapter you want, and you should start with all the weapons you've unlocked previously