I think:
1- I think keys shouldnt be revoked if they stoped beign patrons. They should stay on their itchio account since they already paid for it.
2- For me this is not a problem since patreon charges upfront to my supporters. In my understanding Supporters are charged as soon as they become patrons since I set my campaing to charge as soon as they become patrons. I'm not sure if there's a delay between charging supporters and marking them as supporters. Let me know what you find out.