If I am not a conspiracy nutcase, and Dorion is doing what I think he is doing, he's playing things right for how he should move Karlsson.
That doesn't mean he should move Karlsson....just that if he has to, he is playing the right cards. That is by not shopping him, but putting it out there that he'd move him if he got an offer he can't refuse.
Karlsson gets the biggest return after July 1st of next year because teams acquiring him could be given permission to talk to Karlsson's agent about an extension prior to pulling the trigger on a trade. That same thing cannot currently be done, so teams would be taking a risk that if they give up major assets for Karlsson prior to July 1st 2018, he might refuse an extension and set them up to lose him only 1.5 seasons after acquiring him.
That's why it makes sense for Dorion not to actively shop Karlsson right now. He won't get a better return than he would after July 1st 2018, and by actively shopping Karlsson now he would also risk alienating Karlsson and completely shutting the book on extending him. Even if Melnyk has dictated (or Dorion for that matter, but 'lol') that they will only sign Karlsson for a hometown discount (let's say 9.5M per season) and nothing more....whose to say Karlsson won't have a change of heart between now and July 1st and reluctantly sign that kind of deal? \
Dorion can still listen to offers now and maintain plausible deniability about shopping Karlsson. There's no downside other than the minor risk of a career ending injury.
I will say, trade deadline 2019 is not the time to trade Karlsson. It's too big of a risk because a season ending injury would mean the Senators would lose their opportunity to trade Karlsson, as he'd have zero incentive to sign a contract in a trade when he's already not playing until after July 1st 2019. If the Senators trade Karlsson, they have to do it before the puck drops for the 18-19 season if they want the least risk and the best return.