there's a hole in this:
[...]
cur hands his boarding pass to purebred. purebred boards then on the app cancels her domestic leg (or just no shows, whatever).
Which is exactly why certain (destination) countries have requirements that say that the boarding pass must be matched to the passport at time of boarding.
If the destination has such as rule, UA will comply, and it's why you frequently do need to show a passport when boarding an international flight.
Add to that, CBP do carry out random checks on boarding. After you walk into the jetbridge you'll find them waiting (with no prior indication that they were there), and you'll have to show your passport/BP. It's not common, but it happens.