Thanks very much to everyone who weighed in to try to sort out this situation. I thought I'd provide an update.

Despite some advice from me to the effect of routing through London instead, the parents kept her on the Malaga-Amsterdam-Calgary-Seattle-Portland routing. And thankfully, she made it.

For future reference (though I hope that no one ever has to repeat this experience), before she took off her parents somehow reached Canadian and U.S. immigration authorities in Calgary, who assured her that there would be not problem. BUT she nearly was not allowed onto KLM Amsterdam-Calgary flight. She was the only American on that full flight, and the KLM gate and supervisory personnel were concerned about her taking it. She nevertheless handled herself well, apparently, assuring them that she'd been assured that the transit through Calgary was ok. In the end, they had her sign some kind of paper to the effect that she accepted responsibility for a 14-day quarantine in Calgary if the authorities there required it. It seemed as though the entire flight was held up for 30-45 minutes while this matter was sorted out. But it nonetheless arrive on time in Calgary.

Then, in Calgary, while checking in for her Alaska flight to Seattle, she casually mentioned to the gate agent that she was coming back from Spain. Again, some concern on the part of the airline personnel and lots of discussion before they let her board. She was one of three people on the plane and was upgraded to first class.

The bottom line to my mind, then, is that she and her folks got lucky, and the young woman tipped the balance by advocating quite well for herself in Amsterdam. But again, thankfully, she made it home. Thanks again for all of the help.