OP asked "I know that when I return to Thailand I have to show immigration both new passport and old but my question is where should I let them stamp the entry stamp?"
Like the staff at my consulate told me, you can't get a new passport without cancelling the old one. Once your new passport is cancelled, they give you your new passport. The transfer and the validity of the stamps is separate from the validity of passport, though.
You're a genuine tourist, and look like one in your passport and visits to Thailand (left in between short visits). No need to worry at all. Have 10K baht equivalent in cash, proof of onward travel and a hotel booking if you want to keep it on the safe side.
Still, not official yet. Should mention that it's in the works, and will possibly happen. If it doesn't go through and he uses your advice to book a flight 15 days too late, he will lose a bunch of money.
METV is best for long-term plans ("as long as we can"), which you can only get in your home country. Even if you think you might not need all the re-entries of the METV, getting a SETV from an embassy in a neighbouring country, like you are thinking about, is a hassle, and will probably end up costing you more in the long run in travel costs. And getting multiple SETV's is more and more difficult, the more SETV's you have in your passport.
The main difference is that the SETV's enter before date is irrelevant once you enter, whereas it's still important for the METV, since the date is crucial to get the last border run to get 9 months. That's the part a lot have difficulty figuring out.
