In theory, they're going to fly him in, give him the tour, meet the dudes, etc. and try to sell him on staying for a year.
In reality, I'm not sure if they're not just paying lip service to doing all that ... if they're really in on Duchene like some of the rumors say it's not likely they have real interest in keeping Marleau around for $6 million.
The $3M signing bonus is a given for Marleau this season. Buyout or play Carolina pays the signing bonus.
The $6.25M cap hit is a given because the contract is +35. Buyout or play, Marleau counts $6.25M against Carolina's cap next year.
Carolina either pays another $1.25M for Marleau to play this year, or $416K this year and next year to buy him out. So playing would be an additional $800K this year, probably less than the replacement player for Marleau would make.
No reason not to try if you think Marleau can help the team, whether they talk to Duchene or not. But I doubt it works, Marleau will ask for the buyout, Carolina will comply, and Marleau probably ends up in San Jose on a veteran minimum with incentives.