Good question! The only variety of hydrangea listed in a CSU extension fact sheet for growing outside below 6000 feet was hydrangea arborescens. It requires shade to part shade and moderate to high moisture. You still may want to experiment with this hydrangea. Repotting is a good start and you can take a look at the roots in the process. If any fertilizer was applied to this plant it, it is possible to burn the roots and cause the leaves to turn brown. From the pictures and your account, it does not appear to be a fungus or bacteria causing the leaf symptoms. However, you will be adding more stress to the plant when repotting. So keeping it away from direct sunlight and making sure it has adequate moisture will be important. And it is never a good idea to fertilize a stressed plant. Also macrophyllia usually blooms in summer. You could try pruning to see if it stimulates new growth. Then again, you might "just let it go".

The plant has not been fertilized, only watered. We're on well water too, and we don't have a water softening system, so I've tried to use bottled water (not distilled, just plain 'drinking water') to keep elevated levels of calcium chloride out of my plants (I've had luck doing this with other houseplants). How would I go about pruning this, as none of the leaves seem particularly healthy, and I'm not certain where I should make the cuts? Any potting soil, other than Miracle Gro, you would recommend? Too much fertilizer in that stuff, and fungus gnats.

For now, I'd just snip off the blossoms and any dead leaves or stems so the plant can put it's energy into staying alive.  

As far as potting mix, look for one with good organic matter content, (peat and/or compost) and with little or no added fertilizer.  

I hope this helps, it's certainly worth trying to save it.  Let us know if we can be of more help and  good luck!