burmese are the ONLY country which get just 14 days on arrival and they only get that if they come in by air. They can't even legally enter thailand by land without having bought a visa from a thai consulate before they try to cross
You can get a re-entry permit at the Chiang Mai airport as long as you're stamping OUT at that airport (not just flying to a different airport in thailand to fly out of the country at)
Hopefully someone from the Chiang Mai area can post what's required at that airport to get a re-entry permit
no issue at all, cancel your ED extension, go to the country you're going to apply for the DTV at, apply, get the visa, come back in on it and you'll get stamped in for 180 days