Sharing Ginny as an NPC sounds like a decent plan. I think also with your good ideas for Ron's slow decline into become a darker shadow of his former self, would mean that we would have to set a person to play as Ron, seeing as he's now going to be the antagonist and perhaps, maybe a bit more in the spotlight than other villains, since we'll probably end up focusing on events that lead to him becoming a more left out, paranoid person, and then eventually into events that cement his beliefs that Harry is a bit more reckless and being around him isn't secure for people like, say, his sister and his brother. He worries for Hermione too, at first, but a part of him is still able to connect the dots that Hermione is loyal to Harry, and that he can't do anything for her.

Man, thinking about it, Ron distancing himself and his eventual betrayal is going to hurt everyone. I mean obviously, it's going to hurt Harry and Hermione, but it will hurt other people too, even his family as Fred, George and Ginny are all friends with Harry and Hermione too. And when Luna mentions it in passing to Draco, he would be a bit confused as to Ron's actions, and by the Fifth Year, when Ron is acting as a Death Eater, that will be when he maybe starts to change views, though I'm not sure his views will have changed enough to make him become a member of Dumbledore's Army. That would be up to you. 

If he isn't, then I imagine he wouldn't join Dumbledore's Army but as a member of Professor Umbridge's Inquisitorial Squad, he'd be surprised to see just how far they've gone. And if his views are changed, I think Luna would bring him to the first gathering before they sign the papers and stand up for him, claiming that they are being just as shallow as the Death Eaters, only with the Slytherins, rather than half-bloods.

Anyway, whenever we've decided who plays Ron, I am happy to start whenever~