Under similar circumstances, involving Lufthansa, I simply wrote a letter to LH explaining the situation, asking them to reverse the credit for the flight, and to send me a letter confirming this. I then sent that letter to UA and received credit. The whole process took ~ 8 weeks or so. (Also, I kept detailed records and provided both LH and UA with plenty of supporting documentation including boarding passes (or copies), ticket receipts, mileage summary printouts, etc.)
In your case I would contact SQ and see if they can reverse credit the flight(s) you want to credit to UA?