Retina, 113 81 80 77 70 bytes

M1!`.+[A-Zz]|...s.s?|M[io]n?t?|[AFIOUWN][^o]v?|T..|.*
\B.*(?=.)

T`l`L

Try it online! Includes all 51 test cases. Saved 32 bytes by appropriating @JörgHülsermann's regex that finds the second letter (with a tweak to support DC; edit: saved 1 byte thanks to @JörgHülsermann). Saved 3 bytes by switching to @SteveBennett's regex from his comment to @HPWilliam's answer. Saved 7 bytes thanks again to @SteveBennett. The other letters are then deleted and the string uppercased.