1)
Area51 has ROMS for the Prevail. I haven't checked there since the 2 came out, but they probably have at least 1 for it.
2) All you need is whatever the instructions for the ROM tell you you need - probably Odin.
3) Rooting the stock ROM when you're going to replace it with a rooted ROM (which is just about any ROM you'll find) is a waste of effort.
4) All computers work the same way. The computer doesn't open a data file (document, movie, music, etc.), an app (or program - same thing) does. And you have files that you have no app to open. The solution to that isn't a ROM - you'll still not have the apps you need to open those files. The solution is to find the apps you need to open those files and install them.
The reason to flash a new ROM is to get something not in the stock ROM, without losing anything from the stock ROM that you need. Flashing a ROM just to have a custom ROM is like repainting your car the same color it is. You have a custom paint job, but so what?
If you want the phone to look different, try a few new launchers. Change the wallpaper. You don't need a ROM to make the phone "yours".