There is a tax treaty between UK and Thailand, but I know nothing about it. A quick AI App search did not role out him being liable for tax, if he leaves before the 183 say 175 he should fine. Only a tax advisor could confirm if hes concerned. If he came in so hes been here 180 days he should be fine as well
Two or three months office dependent, before you submit your extension. You should then keep during the 30 day (or what ever) consideration period before you get the final stamp.
this is only a Bangkok bank requirement. I think if you hold the funds for 4 months before, that is it, however. If you hold less than 4 months its then held for an additional 4 months. So it could be 4 or 3+4 2+4, and so on. This is not all braches.
Well they may wonder if your getting what ever visa to stay here longer term, and not having a long term visa, only you can convince them otherwise and only the immigration officer will decide an know the outcome.
If you travel say 4 times a year, fly in/out, book hotels, have a return ticket home, thats fine on visa exempt. Its when you are trying to live here without a proper visa by taking your full visa exempt days (60 days) then extending (30 days) then doing a border run to start again. You will run into trouble. The Immigration officer will be looking to convince themselves that your either a real tourist or someone trying to stay without a visa.