And again, back to football...
You can short change Manziel's improved play and conclude that he isn't any good. Fair enough. But the Browns are an NFL team with a 1st round pick invested and have been completely incompetent in every aspect of the organization. You have to see what you have with Manziel. It's a business decision, not a personal one as Pettine (which you laud) has made it.
I'd ask this question: what difference does it really make, to the Browns, if/how well Manziel can actually play QB? They already know what they have: an immature, unprofessional, unfocused, possible addict that they can't trust.
If they give him some meaningless starts here at the end of 2015, and he plays well, then what? Is the trust going to be magically re-established? Are they suddenly going to feel comfortable with him being the starting QB and the face of the franchise?
If the answer is no (and I think that it is), then there's no more "seeing what you have" left to do.