First: Chiang Mai is a lot more hectic than you've probably heard. I used to live there, and still visit regularly (I'm now near BKK), but my friends in CM are all retreating northwards to escape the traffic and pollution. I'll be visiting them in bloody Burma soon.
Second: whether or not you need to know grammar and such depends on what kind of teaching you expect to be doing. If you want to be able to handle the full range of potential students, then yes. It will be expected of you, and being caught out by some matayom hotshot may well undermine your credibility. But if you teach kindergarten, or "conversation classes" you can probably get by on the notes in the Teacher's Books. The better employers will expect you to know how to teach your presents and pasts continuously and perfectly, though.
Third: I've no opinion on any of the local TEFL training schools.