I'm not sure of the legal requirements of a guardian, I always thought it was one appointed by the parent regardless of age but that's only my belief. If it's right, then there's no reson John souldn't have been Guardian to Sarah, a minor in those days at 19 but not by today's laws.
Your William could well be the one. He was buried at St George's 19/4/1782 aged 62 and buried in the family vault. That would make his date of birth around 1720, though ages were decidedly elastic then. There is no burial for a Martha Finnis but there is an entry for 'Mother Finnis' on 24/2/1799 aged 76. That would make her birth date also around 1820 and a candidate for William's wife Martha.
Regards
AJ