If they were opposite waves (One negative wave, one positive wave, emitted from two points opposite on the globe) then yes. But if they are both positive, they just add in place where they meet and then travel away as they did before.
Correct, and I saw somewhere on twitter that someone near the antipode did see a huge pressure wave from constructive interference.
If you were exactly opposite the source in terms of distance traveled by the waves, yes?
Only if you're exactly on the opposite side of the world I think?