I think I can name a few things. 1. Lack of balls to play aggresive 2. Playing with a frame that has very little power and is meant for counterpunchers 3. too tight of a string tension. I don't think that 2 or 3 are huge reasons for her poking the ball back, but number 1 certainly is.
I personally feel it is something a little different from this though you make good points.
First of all Jennifer had a tendency to sometimes not go for the lines enough. This made her rallies longer than they rightly could have been.
Secondly her Backhand got weaker with time. She could often be pushed into a defensive position by avoiding the forehand and breaking down the backhand.
Lastly she got tight. I honestly believe the prior years nightmare semifinal (wonderful match but heartbreaking result for Jennifer)
got into her head.
If Elena had not been fighting so ferociously throughout that match (and especially in the 3rd set)
then maybe Jennifer would not have gotten flashbacks of the previous year.