I'd like to know the answer for the first part, I did something similar TPE-HKG-SFO-HKG-TPE a year or so back upgrading J-F with miles. I'm thinking I probably just wasted some miles for the TPE-HKG sector that never became F.

Regarding skipping the last sector, physically it absolutely is possible I've done it before. If you have checked luggage tell the people in YYZ to check your luggage to HKG only - that is no problem. Usually when they do this, they can't give you the boarding pass for the onward flight. Then, upon arriving in HK just go to immigration. I try to be a bit more polite and don't check-in for the HKG-TPE sector, figuring it might be more of a hassle for CX to have a pax with a boarding pass not board (vs having a no-show who never gets a BP).

Lastly, I think if they wanted to CX could probably enforce some sort of consequence. MPC membership is at their discretion, which I think would be the most obvious repercussion if they decided to act against you. I don't really think that'd be clever on CX's part to do that...but just saying, I think they could. I think the chances regardless of them doing anything to you after doing this once or twice are nearly nil though - what if someone has a legitimate reason for ditching their last sector? I've had THAT situation happen to me as well: I was on a ticket TPE-North America via HKG, but upon arriving in North America realized I'd have to go straight to PEK on my return for an urgent meeting. Instead of going to TPE (where I was scheduled to spend a day golfing with a friend) I took my flight to HKG and then connected to PEK instead, ditching the HKG-TPE sector. I had no choice. At that point I think I was in the neighborhood of 75k or more USD/revenue a year on CX alone, and I have to say if CX decided to cancel my MPC membership or book me the fare difference over that I would've been pi$$ed. I already was disappointed I had to go to PEK as it was.