I don't think you'd be able to use liquid milk in place of the dry milk, because it's not that you mix the dry milk with water for the recipe. I believe the dry milk powder provides protein, which compensates for the lack of using egg. Pick up a "tub" at the store next time you're there. Mine stocks it on the bottom shelf under the evaporated milk and condensed milk. Since you use a whole cup in the recipe, you'll be able to use it up in no time!
Regarding oatmeal cookies...you'd have to run the nutritional info to see if it's worth it for you, but you could try substituting applesauce for some of the butter (I wouldn't substitute a whole cup of applesauce for a cup of butter, maybe divide the cup between smashed banana and applesauce or use a little bit of margerine and the rest applesauce) and liquified silken tofu for the egg or just use egg white instead of a whole egg. I made cookies like that for a vegan friend and they turned out good - not the same as if I'd used butter and egg, but not awful.
Hope that helps!