Hmmm, It seems to me you would have to think "Dessert wine" in this case. I would use only the amount of sugar (maltose/sucrose/fructose, etc) that it takes to get your desired % alcohol, let that completely ferment and flocculate, then proceed with racking, chilling, potassium sorbate/sodium benzoate addition, backsweetening and then bottling/kegging followed by more chilling. If you're going to save these for some time however, you should at least cork or keg (Or bottle them in plastic or some expanding medium).
Potassium Sorbate will NOT kill yeast, it will not even stop an active fermentation. I beleive it is used mainly to keep dormant yeast from starting up again.
Lactose may be an option and would probably lend a little creamy flavor to the rootbeer, maybe a cross between cream soda rootbeer ale, but then you would want to also add sodium metabisulfite or potassium metabisulfite to inhibit the wild yeast because I believe wild yeast WILL ferment lactose.
Keep on brewing my friends