They are not stupid. Ask yourself how you can regularly go to a Muay Thai gym in Chaing Mai, 700km away from Bangkok, where you say you live? You apply at Thai embassies for the DTV, and you are assuming they don't know the geography/map of their own country.
From my experience the only sponsorship letters that are allowed are those of dependants applying based on the husband/wife's primary application, not from those not married to the applicant, and dependants based on that marriage. Your mom can't write a sponsorship letter for you. That hasn't been approved anywhere.
If they find that you are not still in the country they can, and do, reject your application. Lots of the time when they have doubts if you are still in the country they will ask for a copy of every page of your passport, even the blank pages. You can take your chance, and risk the bt10,000 application fee if you are wrong.