When does the 90-day reporting period start after re-entering Thailand on a retirement visa?

Yes

Yes

Yes it’s been asked before. When you leave Thailand the requirement to report 90 days ceases. Upon return the requirement restarts as day 1 upon arrival.

If you used the online system it may send you a reminder that your 90 days is due, or you may have some paper in your passport saying it’s due on xxx date. Both are irrelevant. You need to report 89 days after your return to Thailand. Or 15 days prior to 7 days after.

Correct

90 days from the day you re-enter counting that day as day one

Correct. 90 days from arrival date in Thailand.