SQ will only put you in C if and only if your e-ticket/coupons show that you are booked in I class on the SIN-MLE segment. I assume your tickets are electronic, which means you may be able to ask DL to perform a last minute change from economy award to business award on that segment + change fees. You may want to do that prior to the first segment on your itineary, that is before you board the SFO-HKG flight, assuming that's only a few days before you'll be boarding the SIN-MLE flight that will follow shortly. Chances are good that SQ would open additional I class for business class award redemption within 1~4 days prior to departure.

Whatever you do, I know with certainty that SQ will only put you in C if your award ticket shows as such or else they will only be paid Y rate by DL, and therefore, would only put you in Y. The only exception is if you are using SQ miles, then in which case, SQ could put you in Business class award seat from economy at the airport because then, SQ would be the one making the changes and they get their own money as they say it.