I've never played multiplayer, but if teams mean that if one member can't declare war none can, then I suspect Canada's teammates can neither declare surprise wars nor have surprise wars declared on them...which is really poor foresight on Firaxis' part.

(Off topic, but I really like your new avatar.)
This is a game about alternate history. That would be even
worse than the current "other civs can't declare surprise wars" nonsense. A lot of civs in the game have been historical allies, historically subject to other civs in the game, or historically never gone to war with each other, but the game shouldn't restrict them from doing so.
Built-in relationships aren't necessarily a bad thing. Civ 4 nailed this imo, by assigning each leader to a certain leader pool (As far as I noticed, there are three:
"Warrior" (Gilgamesh, Monty, Alex, Napoleon, Louis XIV, DeGaulle, Qin, Catherine, etc),
"Reformer" (Asoka, Gandhi, Hammurabi, Sitting Bull, Frederick, Darius, Mansa Musa,) and
"Expansionist" (Isabella, Joao, Saladin, Justinian, Charlemagne, Bismarck, etc) who all liked leaders from the same group. Warriors and Reformers also ALWAYS disliked each other on sight, while the
Expansionist leaders liked only each other but started neutral towards the others. Unless you played with Random Personalities switched on, of course.
The thing is, all of these relationships were programmed for the AIs. AI-Player relationships are fixed, and each group has always one leader that likes the player (Gandhi, Pacal and Zara Yaqob) and there are several warmongers (Monty & Stalin) which start at Annoyed with +0 relations, regardless of what leader the player is playing as. And between that you have other quirks, like Tokugawa refusing Open Borders unless relations are at least Pleased, Isabella bipolarity whether you follow Buddhism or not, or Suryarvarman II's relations plummeting and rising by double the usual amount when you deny or accept one of his requests.
The Agenda's replace what I wrote in the above paragraph, but the relations between AIs themselves could indeed to be tweaked on a historical basis, so that historical friends have a lower relations threshold for declaring friendships and alliances, or so that historical rivals pretty much denounce each other on sight. That would be nice, though I'm not sure how easy it would be to program that in.