I replaced 1/3 of the brown sugar that is mixed with the apples and into the cupcake batter with Stevia (Extract - in the Raw) and they still turned out fantastic. Next time I might try replacing some of the brown sugar in the frosting with stevia as well.
I don't have a hand mixer, so after removing the marshmallow fluff from the double boiler I dumped it in my stand mixer and mixed on high for a few minutes, then mixed it some more after adding the cinnamon and vanilla. The frosting wasn't runny at all and piped well with a piping bag. Like others, I found that only half of the frosting is needed for 12 cupcakes.
Also, I have made these dairy-free for a friend with a milk allergy using 1/2cup vanilla Silk light soy milk + 1/2tbsp apple cider vinegar in place of buttermilk (allow the soy milk + vinegar mixture to stand a few minutes to curdle before using). They turned out fantastic, and he was thrilled to have cupcakes WITH frosting that had no milk!