There is a OneWorld forum where the people know these things inside out and back the front. You would have been better off posting there.

To avoid crazy routings you don't want simply enter each flight sector separately as you want them.

The messages about carrier usually come up if you are trying to pick a code share and the traffic rights don't permit the code share unless you are connecting to or from the code share carrier for the next/previous sector.

I wanted to fly SIN-NRT on the QF code share but it wouldn't let me do it because my next/previous carrier was not QF. However I could do NRT-SIN on the QF code share because my next flight on the ticket was QF metal to MEL.