High Altitude Baking and More

Sorry should have added the type of cookie. I am making a basic sugar cookie 

I found my question from last year! It has basically the same questions  

Sorry, what's the question about high-altitude baking? There are lots of questions here, so I'm not sure what you're really after? I've baked my regular sugar cookie recipe in Colorado with no modifications whatsoever (I kept all leavening in) and it did just fine, but a lot depends on the actual altitude. You may have to just do a test batch and see how it performs. If it spreads too much, then decrease the leavening.

On your icing question: My personal website (www.juliausher.com) has a recipe for egg white-based royal icing under Blog/Recipes/Icing. 

I use organic sugar in my cookies which is also a bit coarse and tan-colored. It can make the edges of the cookie slightly rough where the cutter can’t slice through the larger granules. It’s not a big deal usually, but sometimes I will grind the sugar until fine in a food processor if I want a really clean edge.

I've used Julia's Royal Icing recipe and  I'm consistently pleased with the results. All the ingredients are readily available.