Why not just use pottasium sorbate to kill the east then sweeten it to your desire?
Because sorbate doesn't kill yeast. It inhibits yeast reproduction.
In this case (and all cases of active fermentation), there are plenty of yeast in suspension and they aren't reproducing at this stage- they are fermenting. Adding sorbate to an active fermentation is like giving birth control pills to a pregnant lady. Not going to hurt much, but certainly not going to solve your problem.
My winemaking techniques are far different than the OPs, so I have no good input on the stalling/pasteurizing angle. It's far easier to start with a lower OG and ferment the wine out and then stabilize and backsweeten to taste. Of course, I don't like sediment in my wine and I don't like stalling/stressing yeast because of the potential for poor flavor results.