Incidentally, as I poked around discussing details of mirror behavior in-house, I ran into a puzzle piece I could have used earlier in this thread. It doesn't change the previous answer but adds to an understanding of the behavior noted in your original description.
As you observed, if present policy mirror will be the operational mirror.
Here's the rest of the list of what steps on what - highest to lowest precedence.
Policy Mirror
Smon Ingress Port
Smon Egress Port
Smon Ingress Vlan
Smon Egress Vlan
This rule applies for the 150 class S-series, 140-180 class S-series and K-series products.
Regards,
Mike