Once she gets her USPassport, she should use her US Passport to exit America, then use her Thai passport to enter Thailand, then use her Thai Passport to exit Thailand and finally use her U.S. Passport to enter America. This avoids any need for any VISAS. Good luck.
everything is done online. Once approved, your consulate that services your paperwork emails you the visa paperwork which you present to immigration in Thailand with your boarding pass.
Your question: “Is there a way to keep my American number without paying $45 a month?” - unfortunately once you cancel service, you will loose that number. But - read here:
Yes and one of their locations is at the Trendy Building located on Sukhumvit Soi 13 with easy access to both the BTS Nana Station or MRT Sukhumvit Station not far from Terminal 21.
Use Expedia or similar sites to seek out the airline that flies business class from the airport you will depart from. If you find that reasonable fare, go to that airline’s website to compare the fare and if ok, book directly with that airline. If any issues arises, working directly with that airline gives you better results that with a 3rd party (Expedia and others).