At rest, the trailing arms (while parallel) are angling upwards towards the front of the car (a result of raising the rear to try and counter poor turn-in and understeer). In roll, the outside trailing arms would push the axle back, whilst the inside trailing arms would pull the axle forward. This should tend toward roll oversteer. Cool, eh?
This would reduce understeer in steady state cornering. The picture for turn in is more complicated. I consider turn in to be the transition before you get to an even pitch and steady state cornering. So before the car has rolled which would be the beginning of the transition it would have no effect. You would likely be braking so the car may be pitched forward while your starting to roll and then this will fight you a bit. On the plus it will help you get power down by making the car looser as it pitches back under acceleration.
When you lay with the roll center height that changes your geometric anti-roll and that effect is instantaneous with lateral acceleration. So it occurs before the car rolls and would be more significant during the transition.
I agree with you that it seems the normal arms are too short. Bump steer in the rear while in a straight line is disconcerting. When the links are not parallel this also introduces anti-squat and anti-dive forces...