I don't think Nadine comes across as full of herself at all. In fact she came across well in their anniversary documentary, more I think that she's not all that interested in being a celebrity. She likes singing and is good at it, but doesn't really want all the fame that goes with it. The failure of her solo career is down to the poor material that she put out, you can be the best singer in the world but if the material isn't up to scratch your not going to have hit singles. That combined with the medias hate campaign against her meant it was never going to happen for her sadly.
I disagree that she didn't want the fame. When she went to the US, she dated 2 pretty high profile Americans (Jason Bell and Jesse Metcalfe). And she appeared on America's Next Top Model.
I think if Nadine or any of the other girls had been offered the X Factor job, they would have jumped at the chance.
Like the girls, she has appeared in the St Trinians movie and various endorsements for KitKat, Sunsilk etc -- the only thing she didn't appear in was The Passions of Girls Aloud.