I feel like you’re half right? Daughters in law and passers by don’t feel Norman French. Daughter is definitely from middle German, yeah? And I am wondering about the professors example because the whole point is the plural only happens on the first word.
That said, I so want to be a part of the last group.
Courts martial.
Daughters-in-law.
Passers-by.
Professors Emeriti.
Secretaries general.
Agents provocateurs.