The UX CR-700 will have ExPlus seating, which the OP will not qualify for. However, because of the ratio of ExPlus to non ExPlus seats, I've seen a lot of people getting free upgrades into ExPlus at the airport because the back of the plane is full. Certainly not something I would bank on...

Assuming IAD-PNS is on UX, I believe that flight would most likely depart from the UX A Terminal Gates. There's a chance that the YUL-IAD flight will arrive in A as well, which would make an easier transfer. I think Air Canada only uses their gates in the C Concourse, which would then involve a terminal transfer from C to A via the new train. Again, not sure if the above is worth $50 more.