You are correct that her comb could be shrunken as a broody. After 21 days I would try to break her though. I think you are right to do so.I have one hen who is often broody. She has gone broody every month for the past five months.
This time around I have not broken her and it has now been three weeks.
I decided to try this because I hoped that it might get it out of her system until next summer if she sat longer. Also the other hens are not laying as much, so there is not too much drama over the nests and there haven't been fights.
Usually my other two Orpingtons get in flapping, biting fights with the others when they are off the nest while broody. Penny is a mild-mannered hen who doesn't like to fight. She will have to deal with some confrontations though once she rejoins the flock. I am now trying to encourage her to spend less time on the nest since it has been about three weeks. She seems healthy if a bit thin. Her comb has shrunken, but I believe that can be normal for a broody.