What are the requirements for applying for a retiree visa in Thailand, and can I leave the country before my Non-O visa expires?

Depends on the immigration office, so you need to check with yours. Some insist the money comes from overseas, others are happy to just see it for minimum two months in your bank account

1. If you want to convert from tourist to 90 day non-O visa inside Thailand, then the money must be from overseas. Nothing else matters, that's a requirement to convert the visa.

If you get the non-O visa from a Thai embassy or consulate before you go to Thailand, then most immigration offices do not require the money to be from overseas to apply for the 1-year extension, so you wouldn't need proof in that case.

2. You can purchase a re-entry permit so your entry stamp does not become invalid when you leave. You would need to return before your stamp ends as the re-entry permit does not add any time to your stamp, and the days continue to tick down while you are out of the country. You would want to be back in time to apply for your 1-year extension.