I always think the reason Rowan falls out with Karen is because Karen hadn't told her anything about Edwin in her letters and she finds out they're getting married when everyone else does (or just after, actually). It feels like a betrayal as the two of them have always been close. When Nicola runs to find Rowan after Karen arrives home unexpectedly she asks whether Rowan knows anything more, presumably because Karen writes to her pretty regularly. It must have been galling to hear The News in the way she did.

I don't see her as envious of Karen's opportunities, she has so much self-confidence and would have made a far more effective Games Prefect than Karen did Head Girl. She can match up to Crommie while Karen freezes. I don't see Rowan as self-sacrificing, I take her at her word when she says she just wants a good job and running Trennels is ready and waiting for her.