You have absolutely no idea what was really going on with Cohen. Like you said yourself, you are not a football expert. What we all might be overlooking about Cohen is that it's possible on several (or many) of the plays where he got great penetration, he was not doing what he was supposed to do. Maybe he left someone out to dry. There were a bunch of plays where he got right in the backfield and made a tackle. But there were also a bunch where he penetrated right away and left a large hole that resulted in a 10 yard run by the opposing RB. For all we know he was sacrificing the scheme (as vanilla as it was) for a chance to shine individually. Maybe Schwartz knew this--I mean, he coached the guy in Detroit. He knows his game.
So yes, at a glance Cohen seemed to be doing great. But then again, he may have been ignoring his assignments at times for a chance to make a big play. Just throwing this out there because while I am also a bit bummed he didn't make the squad, none of us know the real deal, so there's no reason to criticize the front office about this particular issue.