>Also, I can't play games with a trainer. I just ignore the extra 512
>bytes, then load the PRG-ROM and CHR-ROM from there, but the game
>doesn't work. Am I supposed to use them for something?

If a game has a trainer, you need to use it. The game won't work if you just ignore it. I think the trainer is loaded at $6000, I don't remember. It's in the iNES format spec.

I wouldn't worry about "loopy" scrolling yet. The vast majority of NES games out there use the standard method of scrolling (via $2005).. worry about making them work first.