Pet Project - FFR

Almost everything you want to do on your list is considered to be incredibly daunting to say the least. I myself have no idea how to go about adding more items, other than it is going to require you to learn a considerably large amount of ASM hacking (everything there is to know XD.) Adding anything NEW like that stuff (spells, skills, characters) is just not gonna happen without learning ASM.

There is no editor or anything "simple" that can do what you are asking. You will have to not only greatly alter the existing Square code, but create/develop whole new code as well, I assume. As for the music hacking, there is a tutorial on this site, and that is your best bet there.

Event hacking is by far the easiest of what you want to do, and yet it is still incredibly challenging. But I suggest you start there. There's been a lot of event hacking related posts here recently, might want to check a few of those out first to get a general idea of how to go about it.