Ok, CX Reservations Supervisor called me back.

In regards to HKG-PEK, this does indeed book into PE if available (i.e. as long as the aircraft offers PE service). The two original agents were wrong in saying that travel to PEK must book into Econ Class V.

In regards to HKG-SHA Hongqiao, the T&C only treats this as a city, and does not recognize two different airports. Since both CX and KA serve this CITY, booking would be done into PE if available and into Econ if PE not available.

As for Prem Econ Guaranteed Seat for MPC Diamond, I can also answer my own question. I checked the MPC Benefits Chart on the cx website, and now realize that, if I click onto the "green star", a more detailed T&C pops up for that particular benefit. For the purposes of this Premium Bundle Fare promotion, the Diamond Seat Guarantee would not apply. The seat guarantee is for W class only, but the promotion books into E.