UK residency status would be irrelevant as UK would still have primary taxing rights over UK sourced income. You'd claim the foreign tax credit from UK on your Thai tax return still for UK sourced income.
Vietnam Airlines have only ever asked me when I haven't got a return ticket with them when flying to BKK - I live and work legally in HCMC so no issues when flying back there.
airlines do ask but it seems to depend on whoever the check in person is. I fly regularly to BKK with Vietnam Airlines, most of the time I have a return ticket with them so I don't get asked but whenever I haven't booked a return or outward ticket with them I get asked to provide one. I also flew there last weekend from Phnom Penh and I was asked at check in for an onwards ticket as I was flying back to VN with a different airline.
If they ask, you may or may not get away with your proposed answer. I have seen people being forced to buy a ticket on the spot before the airline has allowed them to check in.
varies from DTA to DTA. Some will cover all pension types whereas others will only cover state pension, thus meaning any private pension you have is considered taxable if you're a tax resident.
I would like to think your accountant's advice is based on your personal circumstances as it's possible to be a UK tax resident with far fewer than 180 days - well 182 to be precise.