Just wanted to report that I was able to successfully purchase the flights over the phone with United, at the price I found on Google Flights/ITA. The phone agent didn't know what ITA was, but I gave her the flight numbers and booking code (P) and she found the flights. Phew! Now I just have to wait 24 hours for Air Canada to confirm, since the inbound TPAC is AC. It's annoying that the UA website didn't display the flights, but good to know this is an option going forward.