A visa run is leaving the Country and re-entering after buying a new visa or activating another entry on multi entry visa. A border bounce is leaving and re-entering without a visa, ie visa exempt.
It no problem entering Thailand, the problem is boarding your flight, LHR are sticklers for correct documentation and want to see onward travel. It's happened to me and plenty of other people. Just be aware.
In theory yes but LHR is a nightmare for travel documentation. I know people with Non Imm visas who have been refused boarding on one-way tickets(myself included). You'll be better off with cheap onward travel within 90 days to avoid any hassle.
Most Countries you need the equivalent of 800k Thai Baht to apply for the Non Imm O visa based on being over 50 years old. The Non Imm O is a "full retirement visa" as you call it!