Can = ability; may = permission
Attorneys general not Attorney generals or sisters in law not sister in laws.
My final one for now is 'repluralising'; 'agendas'. Agendum is singular, agenda is plural, agendas is nonsense.
I think 2 and 3 here are examples where the "right" way makes sense only if you know how the word came into English. But I wouldn't want to take that to its ridiculous conclusion, because then we'd need "attorneys generals", because the word order has been retained from French, so why not the agreement on the (original) adjective, too?
Also, if we do the history thing, can/may falls apart, as they both come from words relating to ability ('can' is related to 'know' and 'cunning', 'may' is related to 'mighty').
[Sorry, my inner Word Nerd needs to jump out whenever it gets the chance!]