If you watch Live Love Thailand's latest video, it will answer all kinds of questions like these. Your answer here is that, as long as you didn't leave the airport on your connecting flight, you always put where you've boarded initially, which is Newcastle. It doesn't matter how many connecting flights you have, you don't them down on the TDAC.
I think I'd just renew it. If you go to a main Post Office, they'll do your photo and check and send it. It costs £14 extra to get it all sorted out, but you'll get your passport back in a week or so.
No. From 1st May only. It's like filling in those forms on the plane before you got to Thailand but you just fill them in before you leave home now. It's really easy, so don't worry.