Well, since a substantial percentage of us AG'ers use picnic coolers... either there's no problem, or we're all ****ed.
Can't speak to that particular recipe, but I'd be inclined to do it as-is. I imagine they're formulated at a moderate efficicency assumption, and if you miss it REAL bad, it would be easy to adjust with a bit of DME after the fact.
pH is more determined by the mash than the water (unless the water's real messed up). Buy some 5.2pH Buffer (a tub will last a long time), it works like an absolute charm to keep your mash in the sweet spot. Don't stress about pH.