maybe not a good idea to suggest illegal work. Sometimes the immigration officers ask questions when entering. And just generally perpetuating lawlessness doesn’t help the development of the country and contributes to the corruption when people get in trouble.
Agents can't process DTV. It has to be done through a consulate. The agent is probably trying to make money. If you don't work and don't want to worry about having to leave the country every year, get a retirement visa. If you earn active income from work outside of Thailand, you need DTV.
you cannot do the first 90 day report online. It must be in person. And the system doesn’t always work in general. You may get rejected and have to go to immigration during any 90 day report attempt.
there is no law requiring this at all. The IRS receives your address when you file your tax return. And, zero issues whatsoever. The IRS does not care where you live. The US government in general does not care where you live. As long as you file your tax returns, you will have no problems with the IRS (unless you're audited).