Also, I don't know what Corey Lewandowski looks like, so I'm not entirely sure what to be looking for in the "damning" video, but I don't really see any pushing, shoving, or grabbing there.
Well it's already established someone did bruise her up, so the question is who, as you said. The video clearly shows Corey quickly moving towards her with his arm/hand out while the agent seems oblivious. Given that there were reports of him admitting he did it, I think it's fair to say that there is far more reason to think Corey did it then anyone else.
In regards to Breitbart - I think it's funny that people are resigning there and that Breitbart in general seems to be backing Trump. They are nothing but a right-wing propaganda firm masquerading as a "news" site, so it's no surprise what is going on with that. The last thing they care about is real news, or accuracy, or integrity. So again, no surprise they threw their people under the bus - And for someone like Trump even.