it depends on a few things, I guess... Sometimes some people can use the "affidavit of income" from their Embassy (at least it was possible last August). And I am not sure about monthly money transfers from abroad... Just to point out different possible options, nothing else =)
Visa exempt for Russia's citizens is still valid. It was some really weird situation for a few years when Russia was in 2 lists simultaneously - VoA and bilateral agreement for visa exempt entry. They cancelled VoA. But they didn't do anything to bilateral agreement about visa exempt.
The visa itself is only a "ticket" that allows you to enter Thailand. It does not allow you to STAY in Thailand. But every stamp made based on visa allows you to STAY in country until the date in that STAMP.
So, when "valid until" date passes, you will not be able to use this exact visa to enter the country. But that has nothing to do with your Staying in country =)
Every time you enter Thailand with this OA visa (until it's "valid until" date only!) you get a 1year stamp. And because it's multi entry, you do not need to buy re-entry permit during first year (until "valid until" date).
So, you can enter the country a couple of days before "valid until" date and get a 1year stamp again. BUT! As soon as the "valid until" date is passed, you will need a re-entry permit to go out and back (if you want to keep that last 1year stamp valid).