I don’t think this is a mystery, particularly. The hard-fork code will be merged to the btc1 repo, run by Jeff Garzik. It’s not a lot of code, and there are already several versions floating around from previous attempts to raise the block size.
It’ll be up to the developers of other implementations whether they also implement the SegWit part and the x2 hard-fork part. I doubt Core will, not sure about Bitcoin Unlimited. If the fork is successful — ie the economy accepts it — Core will either merge the change or dissolve itself and the developers who haven’t rage-quit will gather around another implementation. If the economy rejects it, they’ll carry on running Core.