In regards to Cabotage, the airlines do take it seriously as they can be fined for breaches. But sometimes an itinerary can slip through there system.
Many years ago I was traveling on a DONE4 fare. I arrived into the USA on CX from HKG, and my exit from the USA was a week later LAX-JFK-LHR, taking the QF B744 LAX-JFK with a direct connection to LHR on BA. So technically the QF flight LAX-JFK was purely domestic in the USA and breached the terms under which QF operated the flight. Nobody had picked up the issue until the LAX-JFK flight had departed. Upon landing at JFK I was met at the aircraft door by the QF Station Manager who was keen to understand how I had managed to get the itinerary issued.
The QF Station Manager explained the "breach" and noted that if I had arrived into the USA on a QF flight, it would have been ok, even with stopover in LAX. But since I had arrived on CX, they were not permitted to carry me LAX-JFK, but of course if was all too late to change it by the time I arrived at JFK.
I have no idea if any fine or reprimand resulted. My ticket was issued by our corporate travel agent so was not my fault (even though I had specifically requested the flight combinations). I did enjoy the QF 744 upper deck for the LAX-JFK trans-continental service, and earned more SCs than I would have by taking the direct BA service from LAX-LHR. Come to think of it, I was actually traveling DFW-LHR (via LAX and JFK). Why travel direct when you can connect for maximum value of a DONE4 fare - I had the whole weekend to get from USA to UK
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So if you can get the ticket SYD-KUL-PER issued, there is a good chance it will be honoured and not identified as an issue. If it is picked up before departure, than technically QF would need to find another way to get you to your destination, but that may not end well in terms of timely transport.