That should be fine. Where the agent thing is an issue is where they get an agent to do the extension without the correct funds. Usually it’s done in a province where you don’t live and possibly have never been.
There isn’t a limit as such. Each embassy or consulate will have its own rules. For example Penang in Malaysia will only issue one per year, but it you apply for another in KL you will probably get it.
In your home country they will probably issue without question.
Each office has different requirements and “rules”. What may work in Chiang Mai is completely different to say Bangkok or Phuket. That’s why most here will say visit your office and get the specific requirements from that office.
TR visa is for 60 days, not 90. If you got a Non Immigrant 90 day visa that cannot be extended for anything other than a year, meeting the requirements to do so.
In theory that could work. In practice it probably won’t. Many offices will say that you need to leave and come back on a “clean” stamp to get the Non O and subsequent extensions.