Don't beleive AC can get a seat assignment on a codeshare via aircanada.com. Call UA, and if you get a good agent, they can give you a Y seat. Have your UA PNR ready for them. If the agent says they can't do it - call back, and repeat until you find one that can. I recently had this done, though took a while for the agent to figure out how to do it. My guess is the ICC may have trouble with this, so try to get to a US-based call center.

I just pulled up a seatmap for that flight as a UA premier, and see lots open in both E and E+.