What are the financial requirements and grace period for switching from a marriage visa to a retirement visa in Thailand after the spouse's death?

Yes, I'm sure you can stay until the last date and this should be known by everybody who applies for the 1 year Extension of Stay as this is one of the documents needed to apply.

You did sign this:

Can he show deposits (65k+ THB), from a foreign source each month into a Thai bank account for the last year, that maybe an option for showing financials.

You are talking about the 1 year Extension of Stay.

In case the wife dies, this Extension of Stay does not have to be cancelled and the person can stay inside Thailand until the permitted up to date stamped in his passport.

IF you want to stay after that date, the person needs to apply for a NEW 1 year Extension of Stay based on one of the reasons.

This could be marriage IF the person marry another woman

Retirement or any of the many reasons used for the Non Immigrant O visa.

And yes, the person needs to meet the requirements of that 1 year Extension of Stay and in case of retirement the money in the banks becomes than 2 months prior application minimum 800.000 Thb and must stay there for at least 3 months after approval and the rest of the time of the Extension of Stay not less than 400.000 Thb.

In that case, if I understand right, foreign husband can stay until the the end of the current extension. So whether enough days remain or not depends.

@T**

should have a concert answer...