I had ten years with SIA on the -400 and CARGO JOCK is talking embittered tosh, most of his bile is misdirected and the product of his imagination.
The rostering of 'mainline' is a fairly simple procedure. The London based crews have their set patterns that they fly, designed to get the maximum hours out of each individual in their work cycle and yes, it means that getting an 'in-command' trip to LHR is pretty rare for SIN based crews, so every LHR includes Stby in London. The rest of the LHR based crews patterns are fairly ordinary.
The next priority is training and they will be given whatever COPs they need to ensure that training continues and that may include pax and cargo combined to maximise useful sectors.
What is left is dished out amongst the remaining pilots and if it is expedient to use a mainline crew to operate a freighter service to get them in a position where they can then continue to be productive to the maximum then that is what they will be rostered for. The mainline crews are more versatile as they can operate both fleets. Maybe CARGO JOCK joined the wrong fleet? Maybe he didn't listen enough at the interview or ask the right questions?
The idea that the mainline pilots can say to SIA, "give us the best combination of trips, or else!" is just pie in the sky and it doesn't happen. The best they can do is request a COP well in advance, requests are limited. The only governing factor is maximum productivity at the least cost, go ask the people who put the COPs together, they are the ones you should direct your venom at CARGO JOCK and should you have the courage to do so you will quickly see that your fertile imagination has led you astray!.