Thanks for all the great info (and to moderator for moving this post). If there were a rail or similar link into downtown we would probably do it, but I think it's better not to worry about possible traffic snarls, lugging our carry-on around with us, and stay at the airport.
We are flying Air Canada on both flights, so hopefully that makes it easier. Domestic AC arrival at YYZ (in J class for all) and international AC departure from YYZ (2 of 4 in J class, still hoping to upgrade the other two). Since two of us are booked in J class on the departing Paris flight, I'm guessing we will want to use the MLL in the international departure area/terminal? We have guest passes we could use for those in economy, and we also have PriorityPass Membership - are any of their member lounges be better than AC's MLL for international departures?