I'm loving all of this. For my individual characters, what you've got there already is pretty accurate. Here's just slightly more in depth for some...
Nik - would be all about helping lower-form Slytherins escape, and would absolutely be game for fighting afterwards, with or without the use of his powers. Since his form's a falcon, I could kind of see him being useful for keeping a lookout while some of the other people help with the escapes, since birds of prey have excellent vision, etc etc. As for fighting after he'd probably feel more useful as a human being, so he'd be dueling and would likely be injured but make it out alive.
Eloise - She's still recovering and won't be at her full ability but did want to help the DA in some way. She would definitely refuse to leave the castle again - she feels really guilty about making a break for it. I think she'd fight where she could, but possibly have to give up early and lay low. She'd fair better if she were with a group. And I think of my students, if we need some deaths, I'd be willing to volunteer her. Eeee. :/
Su - Definitely fighting with or without the DA. She won't get out unscathed, but she'd survive and keep on til the end or until she couldn't physicall fight anymore.
Argus - Fighting, probably with his ~*~white silk gloves~*~ on. He's quite good in a duel despite his prissiness, so he'll probably take a few people out. Overall, though, I think killing him off would be too merciful, because quite honestly he'd rather die young than grow old and lose his looks and/or rot in prison. I wouldn't mind seeing him get hit in the face with some kind of cutting spell and ultimately incapacitated and sent to Azkaban. Lol.
Fleur - Yup, will be hanging with Bill and helping escapees. Would likely stick with Bill throughout the battle, regardless of where he's going. If he would like help dispatching Fenrir, she would be incredibly enthusiastic to do so, as she hates him and wouldn't mind setting him on fire again.
Orsino has no part of this, he'll probably be sleeping while it all happens. XD
And, REBOOT. I love it. A lot. I think it'd definitely be much more interesting than just going into the next summer, and would give enough time for everyone to regather themselves and get some new plotlines going so nothing gets stale. Sort of like starting an entirely new game, but not quite, right? I do think, though, that it'd be good to have some sort of overarching plot to it. I do love character driven games, but it gets difficult to stay involved after a while if there's no central idea to move towards. This game has been good because we've always known that there was a plot moving towards this battle. I'd really like to play my characters a few years down the road, but I think it'd be more enjoyable if there was a reason behind it, if that makes sense. So, yes, but I assume it'd just take a planning period. :)