Has trading for a coach ever worked out? The only time that might qualify would be the Bucs trading for Jon Gruden and then immediately winning the title. He did it with Dungy's players, though, and never got close to that again. It's just as likely the Bucs would have won that year with Dungy, anyway, because that team was amazing.
And yes, trading for Tomlin would be a quintessential Danny Boy move. I see nothing in the record supporting the idea that Josh would do that. He's going to want a fresher, analytics-focused hire.