@Natalie *****
she can come in "free stamp" if she's from one of the 90 + countries that get 60 day visa exempt entry.
At most consulates you have to already HAVE your year extension based on employment for her to even apply for a "piggy back" Non-O visa based on family of a foreigner working here
SO bring her in free stamp, get the 30 day exetnsion on it, hopefully you'll come in on a Non-B and have the extension before that 90 days for your mom runs out
then send her to a thai consulate to apply for the 90 day Non-O with your documentation