I'd move Carlson out for EK in a heartbeat. The issue here is the Carlson is a UFA and is likely going to double his current cap hit with his next contract. The issue with Melnyk is that he's a cheap bum who lost Turris because he didn't want to pay him and is about to lose EK as well. I can't see the Sens trading away EK for a pending UFA defenseman whom they'd immediately have to cut a giant check to.
The Sens are really intriguing as a trade partner though. Duchene, Hoffman, Karlsson, Stone, Pageau... if GMBM wanted to pull off a blockbuster shake up jettisoning Carlson and/or Backstrom then Ottawa could make a lot of sense as a trade partner.