Desharnais' play as of late has been excellent but yes I agree that he most likely won't keep that up for the entirety of the year. As for Sean Walker he's played very well with Philly so far this season but just like Desharnais I don't expect that level of play to last, Walker is best suited as a bottom pair defensemen in my opinion.
Walker however carries a 2.65 million cap hit and is expiring, with 50% cap retention it will just barely fit into our situation and leave us no room for anything else
By the way a package of a 1st, Broberg and a 3rd/4th is a sizeable package, worth much more than a bottom pairing dman. For instance Mattias Ekholm, a player with a long history of being a reliable #3 defensemen on a good team and also being able to manage top pairing minutes and matchups when need be cost us a 1st, Barrie, Schafer and a 4th, and that is with term and a bit of retention too. Combining a 1st, Broberg, 3rd and Ceci for a legit top 4 defensemen (ex: Chris Tanev) would be a better use of our assets. Just like some of the players mentioned above though Ceci has also played an excellent stretch of games. For the record Tanev is a very desirable asset and also Calgary may not want to trade with us but it was just an example of the caliber of player I'm thinking about.
It's too early to tell exactly what we need, I think we'll have a better visual on our team by the new year.