Sassori, welcome to Flyertalk. I don't have experience with China Airlines, but my family has done the international unaccompanied minor flights for years between the US and Germany. The younger girl is now 16 and no longer needs the service. We've almost always been allowed a gate pass to accompany the girls to the departure flight. (In fact, it's been required for parents dropping off to stay until the flight takes off.)

The Canadian government offers guidance on documentation you may need, though I'm probably not telling you anything you don't already know: http://travel.gc.ca/travelling/children/consent-letter

I like your plan of meeting your boy in New York as his flight arrives. That might do much to alleviate some of the mother's nervousness. With no direct flights between Berlin and Minneapolis, our girls always had a stopover, and that caused a few scares with missed connections over the years. The airlines were good at getting them priority for rebooking, though.

All in all, we've found flying the kids unaccompanied to be a safe and effective way for our girls to travel internationally.