Don't do anything until fall. You may continue to be surprised. Wait and see to what extent it recovers. Before the deciduous leaf drop, you will know what is alive. You could gently cut into the cambian bark layer also to look for green. Then prune back all the dead growth. You will need to train a new central leader to get back your height from one of the most vertical new branches. It can take years to reconstruct a lace leaf JM since they are relatively slow growers. I would feed it lightly to help. the recovery with an acid fertilizer unless you native soil is acid already.

My summers are definitely zone 7. I have a Boskoop Glory that gets full sun after 12 and does well. It turns a pretty orange-red in fall. I've bought a lot of JMs from World Plants as 1-yr and 2- yr. olds that have arrived very healthy and are thriving (if you don't want to spend much and have a lot of time for them to grow up). I also have 2-yr.-old Osakazuki & Hogyoku that are pretty much in full sun and are doing fine. But I do water in dry spells. I have a Bloodgood also near the Boskoop Glory, but its leaves can get pretty shabby by summer's end. World Plants says "Emperor" is like Bloodgood but leafs out later (saving it from frosts) and can take more sun.

Here's another (and simpler) possibility. If the JM is goes for several weeks w/o water during the hottest part of the summer, it may respond by selectively letting some (or most, depending on the drought severity) branches die. This may not become apparent until the following spring, when the affected branches will not leaf out. Although lack of water is the "root cause", it might result in various proliferation of various infections, e.g. verticillium, that may be present in the weakened, dying and eventually dead branch. There's no way to know if this is the explanation in your case, but if your tree went without water for more than a week in July or August, I'd suspect something along these lines.

I've had the same problem with my Crimson Queen and Viridis--some branches with no leaves and overall, not as full as they have been in the past. I'm sure it's due to the streak of weather in the teens and inches of snow we experienced last month--here in Michigan it was in the low 80's one day and in the 20's a couple of days later. The Crimson Queen is starting to put out some leaves on a couple of branches that had no growth on them. They both were just starting to get buds when the big freeze came. I think they'll be okay and will get some more leaves as the season goes on, just won't look as good as previous years in this one.