For 1900 baht you can extend your retirement visa and be able to come and go as you want, as long as you get a re entry permit, that costs 1000 baht for single entry or 3600 baht for multiple entries. This way you avoid any potential changes to immigration laws or the whim of the immigration person at your entry point.
You do not need an agent to do your extension, it is easy to do yourself and avoid the cost. You can find the requirements on the immigration website. The key one is the 800 k that's been in the bank for at least 3 months and has never been under 400 k at any point through the year
Your 90 report is based on your arrival date in Thailand or your previous 90 say report. Visits to immigration that do not involve 90 day reporting do not count as a 90 day report. In the eyes of immigration the 90 day report is a stand alone document
The Manila Embassy says issued 72 hours prior to the scheduled departure time of your first registered flight on your COE. in my case I had 3 flights on 3 concecutive days