As I’ve said previously I can’t speak to what specifically Twitch use to verify who the legitimate owner of a game is, there may not be any specific requirements listed because Twitch may have access to a range of methods and tools available to them to investigate the ownership claim. Also as I mentioned claiming ownership of a game gives access to much more than just setting artwork, so it’s crucial that legitimate ownership can be verified.
I’ve messaged someone at Twitch to make them aware of this thread, and they may be able to assist you with your game ownership request when they are available, and provide more clarity as to what info you need to provide.