I'm with Crutch. Personally, I think it's rather unbelievable for Jack to be doing anything involved with Ascension. I think what could be a viable alternative is having Ascension station under attack, and running into Jack with a group of mercenaries on the station working to protect the inhabitants from Cerberus. That way, the 'biotic mentor' route for Jack is undercut while keeping that thread or plot alive.
It's pointless to tell a story about mercs. It's not about Jack so much as it is about Ascension vs Cerberus. And the reason she was hired wasn't to be a full on instructor anyways. It was a last minute attempt to prepare students for war.
It's very believable in the sense that the Alliance and Council is barely doing jack sh*t about the Reapers... and even keeping Shepard locked up on earth. It makes sense that Anderson or Hackett would be doing what they can.. if they can.. with any of Shepard's associates. Desperate times call for desperate measures. Like Hackett getting Liara to work on Mars in some (unbelievable) attempt to find a Prothean solution. Or Anderson getting Kahlee to employ the most powerful human biotic alive, to train students in a crash course.
Think about it... she's the most powerful human biotic (codex wise). Why would they NOT get her help? The rest of the Alliance sits on their asses, but Anderson and Hackett don't. "Get everything and everyone you can for the cause."
As for Jack's attitude, she cares about kids in the same way Renegade Shep turns to mush because of the kid on earth.
Symbolically, Jack fits because she was always the middle ground between full on alien love and full on human supremacy. She represents humans beaten and bruised, trying to thrive in the galaxy. And more importantly, she's a human who never uses others. She was always this way...no matter how ruthless, she refused to use people, and condemned those that did. That was how she mocked Cerberus, and an overly renegade Shep, and how she ends in ME2 if you try to take the Collector base. "Seriously?! He's a user Shepard. Just like Collectors." And Ascension fits her because they don't use humans to further the human cause.
edit: lol.. it's apparent that I like Jack. Sorry for being a bit longwinded about it. Every character needs an advocate though. 