I think Horford is the guy to trade for James, I'd also include Brown or Tatum as I just don't see the minutes being there for both and I think it will take more than Horford to get the Cavs to make the trade. I'd get back Korver as well as I think he would be good off the bench for his shooting. So Horford, Brown/Tatum, Yabu (or Semi and Nader) for James, Korver.
At that point, I'd try to move Irving and Tatum/Brown as a centerpiece for Davis or Irving as a centerpiece for Leonard. I would be worried about the Irving and James combination and in this scenario you move on from Irving since you are bringing James in. So let's say Irving, Morris, BOS 1st for Leonard and Mills and it is Brown that goes out for James. Boston then re-signs the free agents.
PG - Mills, Rozier
SG - Hayward, Smart
SF - Leonard, Korver, Ojeleye
PF - James, Tatum, Theis
C - Baynes, Monroe
I say bring that on.