I'm not surprised you had issues, as it sounds like you were experimenting with many new variables all at once.
With the cookies browning more than you'd like, that could have been due to at least three things. First, you used thinner parchment paper, which is a less even distributor of heat than silpats. Second, it could be that the oven was running at a higher temperature than the one you're used to. Did you actually check the oven temperature with a thermometer placed inside? Just because the oven is set at the same temperature does not mean it's actually operating at the same temperature. Gas ovens also often have convection (forced air) modes; if it was on convection mode, then the effective oven temperature is usually about 25F higher, and this could also have been a factor. But the browning issue is easy to correct - just watch the cookies as they bake, and pull them when they look right to you. Oftentimes, you also have to reduce the quantity of leavening when baking at high altitude, as cookies and cakes will rise and spread more, but it doesn't sound like that was an issue for you.
As for the icing, I imagine the climate there is a lot drier than you're used to? So it would not be surprising that your 15-second icing seemed thicker and drier than usual. But the end result (grainy icing and some bleeding) is usually a result of icing taking too long to dry, which can be because the drying conditions were too humid or the icing was too loose to start. My guess is you over-loosened the icing when you thought it was too thick, but hard to say without experiencing the exact drying conditions.
Best of luck.
P.S. I wouldn't call these cookies an epic fail. You're probably seeing more issues with them than the average cookie-eater would!