Ok here's my take on the social convention thing.
The original BR rules seem to be working with a concept that it's not so much a question of actual divine necessity that you use a spell to transfer control of holdings, but because it holds great importance to the people that a transfer be done properly and with ecclesiastical blessings. All the investiture spell does, is ensure it actually happens without error.
So; guilds and sources are relatively unimportant to people in a "what is best for us and our nation" approach, which means it's not necessary to acquire the blessings of the local priesthood. It is like today when we couldn't care less who actually owns a corporation, but we do care who the police is ultimately responsible towards.
In a Birthright setting you have the temples as the traditionally guaranteeing factor of a certain ideal, in a modern setting this has expanded to include a lot of organizations and factors.
This leads me to the conclusion that you could eliminate the investiture spell completely and without any harm to the idea of the game, it is basically a matter of rpging it properly. F.ex. Any transfer of land or authority (provinces, law, manor and/or temple), will require the willing participation of a local temple or trouble will certainly arise.
What I don't think is useful at all, is reducing investiture to "just" concern itself with provinces. First of all because it reduces the spell to near uselessness. Second; what's so special about provinces? Should it be because it's the "land"? Because it is a solid element compared to the more insubstantial holdings? Manor holdings deal with land as well, is most likely primarily a depiction of how efficiently the human resources are spent on agriculture and what have you.
Third; Elves don't need investiture spells to transfer control of their land, never have, never will. So it stands to reason that this whole investiture business is in fact a solely spiritual element, it is something that has to do with our being, the mystical element, faith, religion - you name it.
Just like any ceremonies of today. The difference is noone casts a spell that ensures complete success to whatever the ceremony is about.
So, my opinion is: Either keep the spell as it is or loose it completely and let investiture become yet another exciting event-machine as the complete certainty of succes is removed from the action, but don't make it a useless apendix with little reason behind it.