What are the requirements for the 500,000 THB bank balance regarding Thai visas and extensions?

It depends on the embassy you are applying at. For re-entry you would only need to show your visa/evisa, but if you plan on getting an extension within Thailand it appears the original requirements need to be met again. My suggestion, do a border bounce and not an extension.

To be honest, this is a question that really no one can answer. If you read the various successful DTV experiences, you can see that some were asked for 1 month, 3 months and even up to 6 months and also different # of months at the same embassy. You're right , no one knows if it needs to be there for the entire 5 years but if you are going for extensions or leaving and coming back in, there is all possibility that you could be asked for proof of that amount, proof of current enrollment in your soft power activity, etc... To be safe, I would just keep that amount in for at least a month or 2, then apply, if you are rejected on that issue, then wait a few months and apply again.

It seems these questions are specific for the particular embassy/consulate you are applying at.

You’d need to check their website for their requirements.