You can't claim that you already showed that you beat Washington, but then say hypothetically you'd beat USC. Utah's schedule was perfectly set up this year to insulate every difficult game with cake teams on either side.
Utah might have risen up to be undefeated with BYU's schedule, but I don't think so. That gauntlet to start is hard to manage physically and mentally. As pedestrian as Washington is, they were set up for BYU as a scheduled loss. I don't know if Utah wins that game