The third pairing in 15-16 wasn't really a typical third pairing matchup wise though, Yawney/BB were basically rolling out all three pairings equally, didn't even bother matching up the dmen (just the forward lines) and all pairings got equal PK time. I don't buy that Vatanen started struggling in 16-17 because of matchups and QOC, something else changed (whether it was the shoulder injury, coaching, confidence, getting a shiny new contract I don't know) that made him stop being the player he was.
I completely agree with your last paragraph about the timing and what I would have preferred Murray to do. I will give Murray credit on getting somewhat of an ok return for a guy who was struggling but if Sami does bounce back then we've essentially sold extremely low on him and it looks bad for us. If he keeps struggling then it looks very good for us.
You're right that it was not your traditional "3rd pairing", but I definitely think they had more favorable matchups than the other two pairings. My point is that aside from a small stretch with Lindholm; Vatanen has never looked nearly as good as he looked that season in a larger role. That's always the concern I had with him. I just wish we would have capitalized on his trade value after that 15/16 season. I think he would have returned a much larger return.