Does the bellcrank have to be in the middle of the wing.
No
What about moving it inboard
Well, you
can. How far inboard are you thinking?
does this have an effect on stability?
As long as the control system moves freely, the only thing that matters aerodynamically is the position of the leadouts and the ratio of leadout motion to elevator and flap motion.
Usually you see bellcranks mounted more or less in the center of a wing, but on a profile you want it offset one way or another so you can get the control rod out. On a full-bodied stunter you'll often see it offset just a bit so that the control rod comes out smack in the center of the wing.