Perhaps trade Dunn when Gunnarsson returns? Play Ed, Bouw and Gunnar on the LD and hope to hell Mikkola or Walman are ready by next year?
No way. Mikkola has potential, but Walman probably won't be an impact NHLer. For all of his tools, he's terrible at using his teammates and often looks like a circus act when trying to defend. He's the Paajarvi of defensemen. He might gradually clean up his game and figure out how to use his shot as a powerplay specialist in a few years, but I'll be surprised if he ever becomes as good as Dunn is right now.
It's nonsensical to deal a very young sophomore defenseman who can skate so you can play UFAs who can't stay healthy or perform when they are. The Blues cannot move Dunn unless they're also acquiring his replacement.
And if you aren't dealing Dunn as part of a package for an upgrade defenseman, what would you hope to get back for him that would change anything with the team right now?