If you're not in your home country then you cannot apply for the METV from there.
But that is irreverent because you would still only receive a 60 day entry stamp as it's a tourist visa. You're going to be have to do a border bounce to get a new stamp either way.
Your landlord needs to go back into the TM30 system, search for the submission they made for you, and click the export button. This will generate a pdf that is your proof of TM30. This is only available for 7 days after they submit it. It looks similar to this photo but I believe it's been updated so it looks a little different now.
What will be required for your extension is different for every immigration office. Ask your office for their handout of requirements to extend based on being over 50 (often called retirement extension). You can file an extension when you have 30 days left on your existing stamp at most immigration offices. For you, you should be able to file as soon as your bank deposit hits the 2 month mark.
You have not had a stay that can exceed 90 days yet, so you are not subject to 90 day reporting.
Even after you convert to non-B visa, that is only a 90 day visa and not subject to 90 day reporting.
Even you go to get your first 1-year extension based off work from your non-B, they will put a piece of paper in your passport that will give you your first 90 day reporting date. Because the 1 year extension will be the first stamp you receive that allows you to stay more than 90 days.
I don't think the Thai government will be sympathetic to someone willingly becoming stateless in an attempt to obtain Thai citizenship. That process is a last resort for people who have no other option.
I believe it's very far from that simple. Stateless people in Thailand have to go through huge hurdles to get citizenship, it's not as simple as just having a relative that already has it. Half of my spouse's family and many of their friends are still officially stateless without a Thai ID card because it's expensive and in some cases all but impossible to obtain it, despite their families living in Thailand for multiple generations.
And the fact that you already have a US passport means you're no longer stateless.
It's for anyone who can qualify for it. But if having a path to PR or citizenship, the ability to get a job in Thailand, or having a Thai bank account are important to you, then the DTV is not conducive to any of those.