Thank you for reading, and for thoughtful comments! (Always welcome but no pressure.)
Is it clear how the royal family of Siam heard about her?
Sorry to say it's not, or at least not to me. I'd guess either a) they contacted the Japanese government, which said "okay, how about Miss Yasui", or b) they sent an observer to various prestigious Japanese girls' schools who encountered her and suggested her, but I am not sure.