Event Hacking question

It wouldn't be overly complicated to remove the opera scene; the difficulty of replacing it mostly depends on what you are replacing it with. To replace it with an equally elaborate scene to the Opera House may be pretty difficult, but replacing it with a scene where you simply board the airship and start talking with Setzer wouldn't be hard at all.

As for getting two characters at once, it would still fall under event hacking, provided you aren't flat out creating a 15th character. If you just want get Setzer and Mog at the same time, for example, you'd just have to set the variables which tell each them to appear on the airship and in the shop screen, possibly assign their names/properties/palettes/etc, and place them in your current party if need be.