Preservatives are not a guarantee that fermentation can't occur. Sorvates really only prevent yeast reproduction, adding them to an active fermentation won't stop or prevent fermentation.
Having said that, adding it to an already complete fermentation as you plan on doing may or may not work. I would still stabilize with sorbate and sulfite. And then use your preservafied (yeah its a word today) cider to backsweeten without worry.
Meads: Hababero and Sarrano Capiscumel, Show Mead possibly getting split and flavored, and 12 gallons of Bochet Deliciousness
Ciders:3 Ciders with differing additives TBD, Strawberry/Apple Cider
Wine: Black Cherry Vanilla Port