Can I open a Thai bank account with a Non-O visa if I have a Thai spouse?

Pay an agent to help you. I did it in Phuket & was shocked how many forms & steps there were. Without a translator sitting next to me I would have found it very difficult.

No bank would ask you about your marriage status. If you have a Thai partner, official or not, he/she would be a mere translator. Bank staff speak enough English to that extent anyway.

O-visa (can be 90 days),EVisa, plus "yellow book" would let you open a bank account. While doing that from within Thailand, no health insurance (from overseas), no police check report from your country of residence.

That is what I am intending to do: get "yellow book" on the condo I own in Bangkok, O visa 90 days, open a bank account, transfer 800,000 THB, let the funds sit in there for 2 months and apply for a retirement visa.

Common law doesn't exist here, either your married at the umpire or your not !

What's a defacto relationship?

Take your TM30 to Immigration to get a residence certificate as this will usually be required

You can open a bank account on a Non O visa although you might have to visit a couple of branches. Bring your wife and her house book if she has one.