The Sun Stone was the last stone I cleansed. There were a bunch of Reavers working at the stone. So before cleansing it I thought it would be safest to go ahead an kill them ahead of time. (plus, I wanted to fill some black soul gems I had and it was easier to soul trap them while they were hypnotized.)

Question 1: what would have happened if I had cleansed the stone without killing the Reavers first. Would they have turned hostile? I am assuming they would.

Question 2: When I got to the third stone a dragon showed up after the lurker was killed. After the dragon was dead, Miraak showed up and claimed the soul for himself. Is this dragon scripted?

answer 1: No, they aren't hostile, nor are the Reiklings at the Beast stone.

answer 2: Scripted, yes... had the same incident.

Don't kill people at stones before cleansing them, you get the best royal rumble battles if you don't

Answer 1: They aren't hostile. Even when you free them
Answer 2: Yep Scripted, I thought it was Random but whatever.

If you let them live they will actually help you take down the Lurker's that spawns after the stones are cleansed, they did for me at least.