Well, given that most jobs for women at the period in the Marlow sort of class were about 'filling in the time before marriage' (or even genteelly husband-hunting, by choosing particular jobs),possibly with a side-order of 'in case your husband is a wastrel/leaves you destitute', this doesn't seem an entirely implausible course for Rowan. Unlike Karen, she's not been talent-spotted as university material (I'm now wondering about her conscious choice in the matter as opposed to being fast tracked for the benefit of Kingscote's honours board), and doesn't, unlike Ann or Lawrie, have any particular leaning.

If you're filling in time until marriage, and have Rowan's interests, it probably beats being a secretary.

I have also been wondering as to whether it might also be an out from marriage if she is not that way inclined ('the tweedy Miss Marlows' - shades of Agatha Dawson?)