My guess is that Jeremy felt that he could talk his way through the early interviews and ignored Bowler's advice. With a change of solicitor and realising these encounters had not gone well, he was then inclined to take the advice of the more learned barristers. However, I suspect he may have taken some of the advice too literally and ended up denying everything in order to discredit some of the witnesses. This caused him to look defensive and his credibility suffered.
Yes, again you have identified something quite important.
One possible criticism that could be made of Jeremy's defence at trial is that they sought to present the jury with a simple dichotomy: Jeremy is telling the truth, Julie is lying; Jeremy is genuine, Julie is an actress. It's the 'actor or actress' question put to the jury by Rivlin in his closing speech.
The problem with this is that, even if you take the view that Julie was lying, it may not be a very clever defence strategy to say she is lying. You have to put yourself in the shoes of the jury. I state the obvious, but they hadn't seen the case before and didn't have the luxury that we have of considering the case at leisure. If they're told Julie is lying about everything, realistically are they likely to accept this?
It may have been smarter for the defence to suggest to the jury that Julie was perhaps telling the truth about a lot of it, but Jeremy didn't mean it. Or some other permutation may have been possible.
In the end, on a strict view, Julie's evidence proved nothing of value to the jury, even if she was telling the truth about everything.