I'd trade Marner, then once he's had time to digest it, talk to Matthews and find out if he wants to be here long term or not. If the answer isn't a firm yes then trade him also.
I'm sure I will get a ton of grief, you don't trade players of that caliber blah blah blah but we're not winning with these guys as it is now, it's highly doubtful we'll ever win with these contracts, at least not until the cap goes up which probably won't be for several years. Then there's always the chance that another couple years of losing (very possible considering how tough our division is) will make Matthews want to leave anyway as the pressure will be extremely unpleasant by then.
I say this because I just think someone has to go for the simple reason that the team would be better with two forwards who play a playoff type game with the combined salary of Marner. If anyone thinks we can win with half our cap spent on these four guys then I understand they won't agree with me. And Marner's overpaid in terms of cap, that's always been the case, I just keep adjusting the amount he's overpaid by as his performance goes up and down and after these playoffs, it's at a low point. In real dollars he's a bargain though so he's worth a lot to a team that cares more about real dollars then cap space. That's what it comes down to forme - Marner's worth more to a number of other teams than he is to us.