Is it possible to get a reentry permit on a Single entry Non O Immigration visa?

Or does single entry mean only strictly only one entry into the country?

Yes, it is possible to get a reentry permit on a single non o entry, this would keep the original 90 days permission to stay alive, it would not extend that original 90 days.

Yes.

It means one entry using the visa with permission to stay for 90 days. You can exit/re-enter with as many re-entry permits as you want during that 90 days.

1,000 baht for a single and 3,800 for a multiple.