But just don't try to send the unused coupon in to AC to get a refund on any taxes or fees...

It does edge on the ethical, but if you cannot get the return date you want, c'est la vie. Nobody can force you to use the return flight/coupon.

Another suggestion, though, just in case you might have a long weekend later in the winter. Why not book the AC return later in the winter, and the return of the US ticket a few days before that. This would let you have a couple of extra days in the sun later this winter for no additional money beyond your hotel. You'll get miles on both carriers for the full ticket. And no pangs of conscience.