I think the design of the oxygen masks requires lap children to sit on a specific side of the plane as well, though admittedly that's not something that comes up all that often. I know the Embraer RJs with 1-2 seating (which are not what DH flies, I know) have that, and I'd think they'd save putting in the third oxygen generator on both sides for CanadaAir RJs and just require lap children to sit on one specific side of the plane.

I don't know why, but I've had incidents where I selected seats and all the Exit rows were marked as occupied, and then they magically appeared as available several weeks later. I'm not sure exactly what the timeframes and procedures are there to make those seats available for booking.