Well I'm no expert but I thought you might be interested to know of two separate cases in the UK where parents tolerated a juvenile the following year when they nested again. In one case they witnessed the young subadult feeding her parents new eyases while she watched on. She helped the parents hunt and protect the nest, and was seen hanging out with the parents even after the season was over so it does happen. But it doesn't seem to be common or maybe we just dont' know about because the cams and people usually only watching during nesting season.
Urban nesting seems to be showing us that falcons don't always behave the way the experts said they would. I remember reading that the parents drove the young out of the area - and yet I've seen at my own site where the parents tolerated their young for months after they should have left. From what I've seen they leave when they are ready, our female this year stayed until the end of September and hunted daily with her mother. I also read that a tiercel and a female would never have a territorial fight - then we saw one on the live cams in Sussex last Spring. Everytime I read they 'shouldn't or don't do something - it seems they do the opposite!
It will be interesting to see if anyone else has experienced what is happening at your nest site, and also to see what happens. Obviously the fact they are still feeding her shows that perhaps they know she can't get along on her own and she's not a threat as she is their offspring. Now whether they will feel that way when she is an adult will be interesting.
You might also be interested to know that there was a young juvenile male in Hamilton that lost one of his legs and everyone said no way he would ever survive or learn to hunt with one leg. Not only did he survive but he was discovered at a nest site with a mate two years later. There was also a wonderful female falcon at GRE in the US who had a bad leg, one talon was missing and her leg looked like it was turned inwards. She successfully nested and raised young. I remember watching her on cam and she would hop on the one bad leg so it is possible to adapt. Maybe Polly just needs some time to learn to survive with her handicap and once she does - she may surprise everyone. I hope she does