I'd keep Johnson, as I think TB need him more for depth up front, and they're already pretty good on D for this upcoming season, then have a lot of money coming off the books with three Dmen who are set to be UFAs.
They should wait for the TDL and see if they can make a reasonable deal for Karlsson (and have an agreed extension in place). Johnson, TB's top D prospect (Cal Foote?), and TB's next 2 first rounders should be a decent start -- of course it would take more, just not sure exactly what. You know OTT would have big interest in Johnson as he's cheap, signed long term at that cheap contract, and is an effective top-6 forward (but more important to OTT/Melnyk that he's cheap, probably), maybe including one of those UFA Dmen in the deal to open up cap for the deal. So I'd be telling Dorion to sit tight and give him a general idea of what you would give for Karlsson at the TDL, and hope it's worth it to him. Maybe make a bigger deal that includes Stone and really go for it, though probably not realistic with the cap, but just a thought. That'd be my plan, but Stevie is probably a better GM than I'd ever be, so I can't pretend to know better or try to tell him what he should do.
All that aside, I don't think Fault pushes TB over the edge. It's kind of squandering Johnson when they could include him in a package for a better Dman, even if it's not Karlsson.