1. How would an English clergyman in the first half of the 1700s typically describe someone who was of African descent? This appears to be the guy who wrote the description: https://en.wikipedia.org/wiki/William_Cole_(antiquary). My intuition is that the wouldn't just use "black," but would probably use something more specific, like "African" or "negro."

2. At that time, would a "black" person (in the modern sense) have been accepted into the kinds of positions John Michell had (like being a member of the Royal Society), with so little comment on his race?

Black people were very rare in Britain at the time, this term was used very broadly for people like Italians, Iberian descent, jews, etc.

I doubt there is any way to show "conclusively" that he wasn't, but I would say the evidence is pretty strongly against. Someone I quoted in another reply provide more reasoning on what that evidence is.

Swedes were described as swarthy by Ben Franklin, so the conception of what black and swarthy are don't really line up to the modern meanings