Hartnell is a good player that most teams would love to add for a cup run. That being said he is not the type of player a team will trade a young core player for. Him being the center piece for a player like Benn just will never happen. Hartnell is the type of player a team competing for the cup now trades for. He'd most likely get a 1st from a contender and a good but not Blue chip Prospect or maybe a young 3rd liner with some upside.
Pretty much the answer to your question is Hartnell could never be the center piece to trade for a player like Benn.
I don't think Dallas does it... But it's even value IMO... Very even. It'll take overpayment for Benn
First off, it's ridiculous to discuss because Benn is the franchise, he's not moving out of Dallas after they got the contract issue resolved.
Second off, I don't think Hartnell + Laughton + 2nd is too far off, but I still think it's a little light for Benn (and I'm a Flyers fan). "Fair" value is probably Hartnell + Laughton + 1st. But honestly, that trade makes no sense for either team.
I agree with you though that Hartnell + Couturier or Hartnell + Schenn is far too much and is ridiculous. Schenn alone is on track to meet/surpass Benn's 2nd season in the league.