Thanks, all. I was able to re-schedule our Denver-Vancouver leg earlier so we should have plenty of time to wrangle customs without a time crunch. My options were 90 minutes or 11 hours. If I can't wrangle customs in 11 hours, there's probably something wrong. Might be asking for half-day sightseeing tips later!
Something weird, at least to me, on this flight purchase and I fly a lot. Air Canada was expensive, United was crazy expensive, but Expedia was reasonable for the same round trip. I never ever use Expedia or 3rd party site to buy flights, only to check them. Then I usually buy them through the airline site. If anything goes wrong and you bought your tickets 3rd party, the airlines are virtually no help. But this time the price difference was remarkable and just too much, so I bit.
As I endured the Expedia chat window odyssey today changing flights, I checked out all the individual legs in another browser as one-way flights. The grand total of all those would have been cheaper yet! I don't know that I would pull the trigger on something like that, but in hindsight I would have been better off just doing round-trip flights that each converged at Vancouver, and just booked each trip through Air Canada and United, respectively. Learned something new, I guess.