Hacking question

Is possible, via hex editing, to have the game start in the world of ruin and play through those events, and then upon beating kefka have it load up and go to the world of balance? Then playing through the the world of balance, ending on the floating continent?

Yes, though it will take an insane amount of time to do so. You need to become an expert at handling events and getting comfortable at setting/clearing event bits before you even try to perform such a task.

You would also need to rewrite the story, since, in the world or ruin, you start as celes and your purpose is to gather your friends and beat kefka. In your version, you would start with celes, but she doesn't know any other party members, right?

How would you explain that, a decaying world like the WOR becomes a world full of prosperity like the WOB? Play tales of symphonia and you'll get some nice ideas on this one :p