The Skift article linked above suggests it would be illegal. If the DOT didn’t allow QF to put passengers on a codeshare to PPT, then I don’t see how the OP’s itinerary is any safer.
Well, the difference is that OP started journey outside of usa
the overnight thing complicated things, whether DOT would find these as one whole trip or two trips (where the second part is cabotage usa-canada-usa entirely on AC)
Even if it's one trip, i don't know how to describe this , doesn't match any freedom of air # since it's a mixed airline itinerary
note to OP: i think AC has some algorithms to detect cabotsge (eg usa-canada-usa with a short Canada connection), not sure if you'll be tripped up. If you checkin with an agent (eg bags tagged) and your routing is usa-canada-usa, the agent will likely stop you
Be a guinea pig?