There is no financial requirements for foreign female married to a Thai male, but there might be some additional requirements to your Thai husband. Go to your local immigration office to get the latest handout of their requirements and check what he needs to provide.
This is a sample of in-country visa application from the website of Immigration Bureau HQ. What is acceptable could be immigration office-specific(or even whether required or not).
Bring both passport to enter Thailand, and show extension and re-entry permit in old passport to the officer. They will stamp on your new passport based on old passport. Then once back in Thailand, go to the immigration office that issued your extension to transfer extension and re-entry.
First of all, it is not an extension, it's an entry stamp if you enter on the Non-O multiple entry visa just before the "valid until" date.
Your entry stamp is no longer valid when you exit Thailand without a re-entry permit. No problem to apply for a new visa as your previous visa itself has already expired.
Just you know, London has no longer issued Non-O multiple entry visas, they issue only Single Entry visas for those.
It doesn't matter whether you are paying rent or not. But in general, you need to provide documents to prove you are staying at that address legally long-term (not just like staying in a hotel for a few weeks). Usually, it's a lease agreement or legal certification of a relationship with the owner.
You need to check what documents are acceptable at your local immigration office.
, if you mean 65K baht as salary in Thailand for 12 months, no, she cannot use it as proof of financial requirements for an extension based on retirement.