Friend requests should be allowed to be recieved even when the option "allow pm's from friends only" is turned on. That's because sometimes from the player percepective who has friends only turned on I still don't mind to recieve pms from a lot of ppl and I usually turn that option on to avoid spam or to avoid receiving messages from specific people. If others are capable of sending me a friend request I can just accept it then they will be able to message me so will I. I will also say the same thing for guild invites.
Also, there is already an option that disables recieving invites which is mainly used by admins because it's useless for player accounts with the way the "friends only option" works.