1) Available 100 days in advance, I do it by messaging TK on Facebook or Twitter. You can also call but their social media team is pretty pro active, I usually get an answer in less than an hour.
2) No idea
3) At least 3 hours, you don't need to go through security or anything arriving from the US, you will deplane directly in the international departure hall and can head straight to the lounge and then go from the lounge to your BKK flight directly. No security checks.