Yeah, I didn’t consider that. The game would probably have to design the map in a way that keeps all the characters relatively nearby, or introduce a mechanic for them to converge more seamlessly, like maybe having save rooms that teleport all of them together or something.
What, where do you get that? Any publicly conveyed copies of gpl-licensed software must make their source code available, and be published under the same license. This is true regardless of modifications.