For what it's worth: I booked on United last night, called today (888-247-2262) and was on hold only 6 minutes (including prompts) before talking to a live agent who happily gave me seat assignments together for two itineraries.
My AC confirmation number was NOT on the E-mail, but I got it by logging in to United, viewing the reservation and there was a link "This reservation contains partner flights. View additional confirmation numbers." That gave a pop up window with the AC Confirmation number.
The "Preferred Seats" (Economy Plus) were available for purchase. She made it seem like since I was *G I would get a discount but the prices she stated were the same as the regular price online via aircanada.com (I was looking to be sure she was giving us good seats).