One thing I didn't quite understand was what went on with Seven at the end. I mean she was already no longer a member of the Collective and I'd have expected her to have a similar (if much stronger) reaction to Picard's. But the way I understood it she sort of regressed to the time before her assimilation
We don't know that. All we know is that she said her name was Annika Hansen instead of Seven of Nine. Annika Hansen is her real, legal name. Her aunt calls her Annika, and there have been cases in the TNG-R novels where she's been referred to by other characters as Professor Hansen. It's just been her preference to go by Seven of Nine; all we know at this point is that said preference seems to have changed.
And she's different from Picard because she still had implants, and because she was a Borg far longer. Also she's simply a different individual.