Well this one's been sitting for a bit. Uhm.
Yeast won't eat splenda. Or so I'm told. I've tried backsweetening really (really really) dry meads with it and have had mixed results.
Lotta root beer recipes say to leave your RB to bottle carb at room temperature until it feels firm enough and then keep it in the fridge. This is called "Cold Crashing" where if you chill the beverage below the yeast's operating temperature it will stop - or at least slow down greatly - fermenting. People do it with cider all the time. Just saying you could do that with your rootbeer. Thing is, ale yeast is bred to make alcohol, whereas bread yeast is bred to make gas. I think you'd still be better off using champagne yeast as kind of a compromise.
But, standard reply from my sig applies