@Pete *********
and you will note I DID answer your question as completely as possible.
And if you're currently on an valid 6 month METV you can bounce out and back at the land borders without being denied entry. Especially up near Chiang Mai at the Mae Sai - Tachiliek border with Myanmar.
Those officers don't care in the least that you're bouncing out and back so dial the paranoia back a notch or three, USE the METV to get the most time in country out of it THEN apply for the in country 90 day Non-O based on marriage
As long as you still have 21 days or more left on your current entry stamp in Chiang Mai AND meet the requirements you'll be fine..