Well, a few things that jump out at me.
First... the car. I don't see it as all that unusual or undermining or necessarily manipulative for a Dad to give his kid a car. The situation could have been handled quite easily by - as another poster suggested - having your daughter responsible for paying for her own insurance. Situation averted.
$3500 for a custody battle is chump change. ESPECIALLY if it's contested. Try tripling that - at least. Yes, it's a lot of money, but that doesn't mean the lawyers weren't doing anything.
It is entirely possible that the lawyer suggested mediation because he had a feeling that it was the only way for you to get a positive result.
I was less interested in how long the GAL spent with various people and more interested in how the actual meeting you had went. What sort of feel did you get out of it? One thing that I get from your posts - and the GAL likely also picked up on - is that there is a tremendous hostility towards Dad. Now, it may well be warranted. But a custody battle is the worst possible place to show it. And Dad may well have hidden it better than you did.