I had the same itinerary priced by AA and by QF in CAD.
OSL-LHR(stop)PHL-YYZ(stop)-ORD-SFO-DFW-LAX(stop)-MEL(stop)-SIN(stop)-DOH-OSL

The very same flights were used in each case.
AA quoted CA$4535
QF quoted CA$5679 for the same route and flights.

There were some differences in the marketing of flights used
For OSL-LHR, AA used an AA codeshare on BA; I requested that QF do the same, but they would or could not start a Oneworld Explorer with a codeshare, so used the BA flight.
For ORD-SFO-DFW-LAX, AA used AA flights, while QF, at my request, used QF codeshares on AA flights.

1. Would you expect prices to be more than CA$1000 different for the same flights, just different marketing of them?

2. QF quoted CA$5232 (over $400 less if I used SIN-HEL-OSL rather than SIN-DOH-OSL.
Does it make sense that QR flights SIN-OSL would be priced $400 more than AY flights SIN-OSL?