Hi brother H,
after reading your post I have a very simple take at this, and probably not something you want to hear but I say it as I see it:
1. You saw her for the first time and you got FS from her for free, then you tipped her $200, so it's not free afterall.
2. The shop is a Korean B2B and I supposed you didn't do bare back with her, so obviously she would have got condoms ready, why would a B2B girl has condoms handy, you really believe you are the only one she did FS with?
3. If I counted correctly, you saw her 4 times in quick succession, so she was a very successful ML in securing her own regulars. and everytime you had FS, you didn't mention if you tipped her more in subsequent visits, if you did, then she made good money on you, if you don't then she got your repeat business anyway.
4. She asked you to marry her, sounds flattering and interesting, but if you actually say yes, I doubt if the girl will go through with it. I don't think the girl is after your status or money, probably just some no harm fun. may be it is visa related, or she was simply testing the water, you will have no idea how many other clients she has told they are the only person she wanted to have sex with and they are hers!! and how many of them had received her marriage proposal.
5. You have a girl friend right? be nice to her and try to get more intimate with her may be your better bet.
Just my two cents
Cheers