> If something happens and you're unable to renew your domain, are your next of kin out of luck?

I'd say "don't do that". I had a friend pass which I knew had a custom domain for email, I told the relatives they had to be on the ball regarding renewal.

At least my registrar will keep sending invoices for a few months without letting go cough cough, so should be enough time to get the certificate of probate. With that the heirs should be able to get the invoice so they can pay.