Having seen this happening in the past, pre-AI, the answer is usually that they don't. They just stay on that version forever. Or it's maintained until the original author moves on, after which it's someone else's problem to figure out how to move back to the upstream, which has now had years of changes.
Originally, yes. Nowadays, it's trivial. Agents can handle this flawlessly.
There can be a point of divergence when even humans can't merge things anymore, so I highly doubt that experience is universal.
No you can't say that! It's the superintelligence. You will be left behind.
If it's super intelligent... Won't I be left behind anyway?!?