Are there requirements for spending a certain number of days in Thailand on an OA visa, and what are the check-in procedures?

If you had failed to check in 90 day at any point, and you left, when you reenter Thailand, guess what? .. you are all clear. Noted that when you reenter you should do the TM.30 report first, as your newest 90 check in will sometime requires the Tm.30 first

If you fail to check in 90day, the only problem is when you try to renew something they might spot it. Or if you are arrested, they would have a reason to fine you for late 90 check in

There's no requirement to spend x amount of days in Thailand on the OA visa, you are required to report every 90 days that you spend consecutively in Country

None of the retirement visas require you to spend any minimum amount of time in Thailand. You can get the OA visa from your home country and never even go to Thailand for all they care, they already got your money.

The only "checkin" is a 90 day report which is what you do when you have spent 90 consecutive days inside Thailand. If you are not in Thailand you obviously cannot report spending 90 consecutive days. This is for all foreigners and has nothing to do with your visa type (except for one small exception for LTR holders).