Hi Reuben.
Sorry for late reply, I missed the second part of your message where you ask question not directly related to the topic of your initial question. It would be a good idea to create a separate topic in such situations 😉
To answer your question – Stripe says it handles the stuff for you: https://stripe.com/blog/smarter-saved-cards
We have a ticket created for this in our inner system and we definitely will have a closer look into this. At the moment, there is no option to update customers payment details other than cancelling current subscription and re-subscribing again. Since Stripe Payments is not storing any re-usable customer info (e.g. not creating any member profiles etc), that’s why it could be difficult to determine who is who among the customers.