While I agree that I'd much rather have the separation of control only accessible with a double rein, I don't agree that you lose "all" refinement with roundings, nor do I agree that they provide the same effect as a Kimberwick. With the latter, there is no distribution of snaffle action whatsoever, and the curb effect is always full-on and strong when rein pressure is applied. With roundings, you're never going to get the full effect of the curb, no matter how hard you pull. It's always going to be balanced between the leverage effect and the straight pull of the snaffle.
You're presuming that the horse is going to be bothered/confused by the curb, or that it will be too harsh. I'm assuming the opposite--that since the horse has been ridden with a curb rein in the past, it's probably familiar and comfortable with the action, and may even have an active dislike for snaffles, perhaps (or perhaps not) due to the stereotypical heavy-handedness of polo riders. Either of us could be wrong. It's quite possible that all she needs is a tooth float and a French-link. But I wouldn't discount alternatives so quickly.