Many thanks to all for the interesting comments and advice.

It is on one PNR as part of a round trip miles ticket from Europe to MSP.
The risk of AC and/or US customs stopping me boarding combined with the tight connection has persuaded me to rebook on a direct LO flight WAW-YYZ, with the added benefit of saving around $200 in charges.

Cabotage is indeed sometimes overlooked as a potential problem, both for the transport of goods and in this case passengers.