couch co-op is something a game has to implement, not the console. You could do that over with a dedicated pc where all the steam decks remote steam play on it and boom you have 4 remote controllers ( Untested because i dont have so many friends with steamdeck )
Same thought here, why uploading it to proton drive? github isnt evil that they trying to snatch that code or whatever. If you are so cautious then ramp up a gitea or gitlab self hosted instance up.