100% not true, anyone can get a yellow taaban. Contrary to popular belief, even renting a house/condo you can have a taaban, I know many foriegners who have them.
Correct. While Education and volunteer visas are of the Non-O category, normally when we refer to Non-Os we're talking about retirement or marriage. A non-O based on retirement requires 800k baht, marriage requires 400k baht.
All pretty much the same. Whatever you do, don't buy a simcard at the airport, wait until you get in town and go to a shop. Airports only sell tourist sims.
I use Dtac, I find the customer service better than AIS and True, reception wise it's all pretty much the same though. I pay 200 baht a month, unlimited data 4mb/s. Can go up from there.