Everyone is assuming that you are married to a Thai. If so you can get your Non-O marriage visa from the UK by applying online or, after arriving, from Khom Kaen Immigration if you have a Thai bank account in your name and a balance of at least 800k.
Correct. If widowed, your current extension is honoured then next extension you either change the reason or leave. Regarding divorce, that does not happen overnight so, again, you plan your change to the reason for your extension or your have to leave when the divorce takes effect.
My "proof" of my wife returning to Thailand was mainly my lifestyle choice where I spend 3 months in the UK then 3 months in Thailand (UK visitor visa)
The 800k is only needed for just over 5 months if planned correctly (as you have done) then reduced to 400k for the remaining 7 to 9 months of the first year. After that the 65k per month kicks in having aleady deposited it for at least the previous 12 months.