That was a most interesting exercise. I'm not at all certain that so many spirits would manifest themselves, especailly at location but, given their ability to do so, the ending is plausible. 'Twould be mildly entertaining to see how Fudge and Dumbles fare after this, as well as any remaining Deeters who weren't wiped out by that "dance" of Harry's. That one was well written and choreographed.
Couldn't tell ya. I have no idea what a ghost would do. From canon, a ghost is a dead witch or wizard who either has unfinished business, or is afraid of crossing over. (Sir Nicholas, OotP) I'd imagine that Hermione and Ginny would have had somethings they wanted to get done before they crossed over.
I have the feeling the their respective careers are over. Since Harry's 'demonstration' was in front of nearly a hundred reporters from Europe, where neither of them have much sway, any chance of 'damage control' would be minimal at best.
Thanks for the kind words on the 'dance'. I think I'll go to Hollywood and become a choreographer. Snicker!