If I am not wrong there is T&C that stated that you have to fly the flight (not only purchase it). So technically, if you fly on SQ, then you will not get the mileage as you did not fly your CX flight.

I do not know if under this circumstances CX will give you the mileage. Anyway, I thought SQ flight will do SIN-HKG-SFO. Why CX will bump you into SIN-HKG-SFO flight, rather than just SIN-HKG on SQ and thenn HKG-SFO on CX?