I don't even think that they are in cap hell.
Now that Marleau is gone, they have enough space to sign Marner to a reasonable contract or to a slightly inflated contract if they trade Brown. It will be tight and they will have an imbalanced, forward heavy roster, but they can fit it all in.
Now the trick will be to see how Dubas and management can trade from their deep forward group to address the need to improve the D. It also may not happen to start the year, but since they will have plenty of depth up front with lots of good players signed to reasonable contracts, the will probably have the pieces to do it when the opportunity arises. That would have been easier if they had a 2020 first to trade.
Now, if a team comes along and throws crazy money at him with an offer-sheet, then there might be an issue, but if that happened and they didn't have Tavares on the books, it still might not have been a good idea to match a 12M+ offer sheet.
To me, cap hell would be having a bad team with long term, immovable contracts.