1. Do Overseers still have contaminate? If so, I thought they were changing that for HotS.
Still has contaminate. Overseers were originally going to be changed, but that was reverted when they tried detection on Vipers (and presumably other things behind the scene) and it didn't work out.
2. Do Overseers launch Changelings at siege range?
Nope, they just drop out.
3. Why are enemy structures and units unclickable? I hear there is an option to adjust that in the menu. Is that true?
Yes, you can turn it off. I think its just so that it doesn't confuse some new players.
4. Workers now automine. Is there an option to turn that off?
Believe so, but its just at the start of the game, it doesn't even help much, and they don't split.
I hear the worker A.I. does that now due to the way the Protoss Oracle blocks mining. Is that true?
No, sending workers to mine at the start is not as a result of the Oracle.