I don't see why the Mathews situation would degrade to anything worse than the split he had with Tolbert last year. Mathews averaged 17.2 PPG in that time share.
However, the player that was completely undersold in your run down of the trade and actually makes it close is Doug Martin. He could have a very productive year in PPR formats.
It comes down to Mathews and BJGE vs. Martin and McGahee, and it's close.
If you're truly concerned that the Mathews situation is going to be a value killing split, you are getting decent value back. I'm a believer in Mathews. I think holding on to him offers the highest upside. He was a first round pick in early drafting leagues for a reason. However, as you brought up, there seems to be more risk involved now that there has been unanticipated carry sharing.
To recap, I would want the Mathews side of this deal. It's arguably the riskier side, though. On the other hand, if you were to accept this deal I couldn't critique it in terms of value.
Hope this helped some