Can I open a bank account in Thailand with a 90-day retirement Non-O visa and leave before applying for a one-year extension?

Immigration isn't the hard part of your scenario. The bank, one Big Bank in particular, has been making up their own policies and blaming Immigration. Even on an O visa there have been reports of people having a hard time opening the bank account there. The latest fun was asking for 4 forms of ID to open or keep the account. Visa, thai drivers license, license and passport from your country of origin etc.

There have also been reports of people doing exactly what you propose, using a 90 day O to get everything setup. Then leaving for awhile and getting another when they come back to settle. One thing to remember is that working on the retirement O is not allowed, even remotely.

Yes very possible.

Yes, you can. I have had many Non-O visas without getting a one-year extension. Avoid Bangkok Bank.

If you land with your Non-O

Not much point having a bank account ?

You need to put 800K baht into it as soon as you arrive ( minimum of 60 days / 2 months )

If you're doing it just to get a thai bank account, if the bank finds you dont have a long term visa they may close your account as I understand.

Yes you can open a bank account on a 90 day Non-O visa with a certificate of residence from immigration and a Thai SIM registered against your passport. The question is why would you if you are simply going to leave again.