KH: Yes, you are charged the inspection fee, even if you are transitting. US airports don't have sterile transit, so you cannot get around the Customs and Immigration inspection.
BB: Even if you interline the YYC-MEX, breaking the sectors between different carriers, you still get caught on the fare rules.
The fare has to be built YYC /O IAH MEX /O IAH YYZ (for example). OK, so far, but that confines it to one ticket (you cannot break a point-to-point fare across more than one ticket). With a J fare, you can put the outbound and inbound on separate tkts, but you can't break the tkts at the stopover point.
Also, interlining means that the tkt has to be a paper ticket (unless it interlines AA/CP, which lets the agent see the whole PNR history, defeating the purpose of interlining).
The receiving interline carrier at the intermediate point (i.e. IAH YYZ return) is entitled to confirm that you have used the intermediate sectors from stopover to turnaround and vv.
I would figure a pax checking in for a flight at an intermediate point in a fare build without evidence of the interlining is a red-flag for fare fraud, especially on a OW ticket, or a RT with the same /O on the outbound.
I have certainly been asked to show evidence of using the inbound in cases where I am checking in after a stopover and I came in with a different carrier. Given the increasing vigilence of carriers to things like back-to-back's and hidden city tix, I suspect it will get increasingly difficult to do.
All of which leads me to believe that the very best thing to do is follow KH's plan, take some time off, and hit the beach somewhere warm
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