It is usually a matter of reason to return, for schengen after being refused I worked with an agent that created the application along side us. It worked after adding more things.
Showing reason to return like a job, a property, family, car etc... has a big impact. Also showing I had a thai bank with lots of cash apparently was a good thing, the idea being I am invested in Thailand so no reason we wont go back together. Everything you can think of as a reason she wont just stay there is good and make that in a proper dock with a proper header. Add pictures if relevent, my gf work freelance online so no work paperwork but we made pics of her selling stuff on stream and it worked.
Now that was all for schengen, US is probably more stingy.
You should be fine, prepare a wise account in advance unless your bank does not charge fees or cheat you on exhange rates.
If all goes well you can transfer the 400k within 48h technically so your are not that short on time. Thai bank can be opened within the day, a bank or wise transfer can be 1 day.
I recommend you ask a driving school to help you get through it if you don't care about a couple of Ks. Residense certificate and medical is easy to get but registering, going to the presentation on road safety, passing the tests etc.. is all in thai and it is a bit painful without a guide. The writing test is in english, do online one found on google they are very similar until you get perfect score and you will pass.
Kinda yes if you are from the country list for 30 days on arrival, you get Visa exempt for 30 days then extends it at an immigration office to 30 more. Extension can be fast and easy for depending where for 1900thb, do not do in bangkok or phuket or pattaya. It is a pain in the a$$ in busy immigrations.
I think you need to call the airline, although Indigo is pretty aweful... try to get Indigo staff from the airport of departure, customer support do not always know these things.