Hi Goober,
Joanie asks some very relevant questions here, which would definately help with subsequent advice given. As a general rule, Japanese maples do not like intense sunlight. They can scorch easily especially in higher heat zones. The plants tolerance for sunlight will vary from one cultivar to another.
From what you describe, it does sound as though the tree has been a bit nuked by the old sun. Both my Kiyohime and Deshojo live under a shelter and receive no direct rays, and grow well as a result. On the otherhand my Sango Kaku has sat in full sun all summer with no burnt leaves - even on days like today where my thermometer read 34c /94f in the shade.
I would be quite suprised if you have root rot, as your soil mix should be very fast draining and allow for good aeration too, although erring on the side of caution, I certainly wouldn't rule it out. The rot occurs when the roots die, usually suffocating as a result of sitting in waterlogged soil for too long.
I might also ask how often you have fertilized the tree, as overfeeding can damage roots and as a result would cause leaves to dry and softwood to wilt.
For now, I would place the tree in the shade, put the pot on a couple of bricks or something so that air can circulate beneath it, and pay more attention to the watering. My maples "tell" me if I have not watered them enough, as the leaves lose their rigidity. Really though, in this heat and in a fast draining soil, it is pretty hard to overwater a maple, as they are a thirsty tree.
All the best,
Aaron