When you go "home" get a new passport. You will keep your old one and the extension/re-entry will still be valid on your return. It will be far easier/cheaper to renew your passport in the UK than doing it through VFS.
I used to fly BKK>>LAX regularly on the TG A340-500. That route didn't stack up economically I believe. There's no issue with flying the distance, SQ flies further, I think the issue remains economics.
Absolutely! Anyone tries to staple anything in my passport I try to stop them. If they succeed I carefully remove it as soon as I can. These are the very same people that will later make a fuss about a "damaged" passport.
At BKK it's in the immigration section before passport control after you pass security. Down the escalator from security and turn right and you'll see the desk ahead of you. Yes, you can get a multi (I did it recently).
Maybe just my impression but most of the reported problems seem to involve funds from a US account and/or Bangkok Bank. OP has nailed both of those perfectly.
It's a very good fallback and better still requires nothing more than your passport (and money). No copies of anything, no form filling, no photo. You just have to remember to do it!