I'd be interested in any stats that suggest that Duran's effectiveness decreases after back to back games. I don't have anything to base this on, but I feel like Duran has played in back to back games a lot this year... and I wonder what type of impact that has had on him.
I also think your spot on, regarding Gray... and really the only thing I can think of is that he has had some bad games this year. I don't think they trust any starter. But you have to take pressure off your bullpen at some point, and the best way to do that is allow your SPs to pitch later into the game.