I am also curious what makes this dry faster. The only meringue powder I have ever used is CK, which I order from CountryKitchenSA ($10 for one lb. or so). I used glaze for years because I had heard meringue smells bad, and royal icing tastes terrible. Then I saw a post by SweetSugarBell on her blog about how brands can change the taste. I am very picky about taste, but it smelled sweet and like valilla, and the icing made with it was delicious.
I can't see how whipping the mix does anything but fluff it up and give you a larger volume of icing, plus make air bubbles a problem. When I whipped until stiff, my RI transfers crumbled, and weightd almost nothing. I went over to the Montreal Confections blog because she does a lot of RI transfers, and in her icing recipe video she also does not mix until stiff. After all, if it is normal for water to seperate and settle in the bottom of the bowl, it makes sense the same thing would happen on the cookie after a couple of hours. Plus, by whipping until stiff, you need even more water to thin it to a good consistency. I like my cookies soft-ish, and don't like the icing to be rock hard. If I can stack it, it's good enough for me.
I wonder what you tried that did not work? I have been trying to find a recipe that works for me, and it would be nice if I don't have to make all the same mistakes you must have overcome. I am going to give your recipe a try for my next batch, so thanks!