Whedon confirmed in an interview later that Thanos will have a part in A2. Sounds like a small part, possibly leading up to him being the main villain in A3.
SDCC has been changing their ticket buying proccess every year for the last few years. The convention has grown by leaps and bounds just in the time we've been going (this is our 5th) and tickets are gone rather quickly.
Last year they did a presale, only open to those who attended the previous convention. They did that in August. The "open to the public" sale I believe was in March or April? That doesn't mean that they'll do it the same way this year, but I have read that there will be a presale open to previous attendees not long after the convention. So that part at least sounds the same.But you do have to sign up for a "membership" or something. And if you sign up for that you'll get email updates. I already have one, so I'm not sure if you can go to their website and sign up for a new on right now or not.