Starting my very first FF6 hack and had some basic questions.

Everything is so rigidly arranged, and it seems like there's almost no room for alteration. How does somebody even begin to rearrange the game to create their own events?

Event command B2 let you call a subroutine. As an example B2 00 00 31 would let you call the event code at FB/0000. The high byte is calculated from CA/0000, meaning you can't have event code before CA/0000. You just need to place the overwritten code in the subroutine you call, put the new code and end the subroutine with a FE.

Assignations of event (NPC and event triggers) with FF6LE can only be done from banks CA to CC, meaning the B2 command is more than useful for new events.

As for the ROM header, some patches require a header, some doesn't. It depends.