Well assuming the proposed lineage is correct, then this William would have been about 40 years of age. Adding in the children he had with his wife Susanna, he had at least two or three decades left after this point so I don't think debility was a factor.
Though thanks to the help of yourself and others in this thread, I think I now have a fairly solid throughline connecting these documents with one individual. I believe that this is a great-granduncle of mine, older brother to my 5x great grandmother Mary Smalman.
While I need to do a bit more digging, there's a good chance that these two, along with another sister Martha (from whose wedding the last signature came from) would then be children of a William Smalman and Elizabeth Jefferies (m.1727). I'm not 100% set on this link as William the younger is listed as being born three months before the wedding, at least in transcript, but if true William and Elizabeth would be my 6(!)x great grandparents. It's entirely possible that this elder william is the Churchwarden from Monkhopton in 1754.
So thanks once again everyone, you've helped me put together a lot of very tricky puzzle pieces!
EDIT: Having said the above, ciderdrinker posted a will in the other thread that suggests William the younger died sometime in 1776, that'd mean he only had nine years left - maybe you were on to something, Goldie!