Hi Grizzlywon, looks like a bunch of your trees are dying at the same time. There should be a common denominator in all of these cases. What did you do to all of these plants at about the same time? I would guess it is the fertilisers. The signs and symptoms of all of these trees point to root burn. Try not to fertilise in the hottest part of the summer. Even the slow-release fertilisers are not safe because they are temperature dependent and tend to release too fast when it is hot. I can see that those fertiliser granules are all over the soil of your plants. You might have overdosed them too!
The best thing to do now is immediate repotting. Try not to cut off any roots, unless you can see that they are clearly rotted. Your plants are small and their root volume is small, so they don't have any good roots left to spare. So it is safest not to cut any root now. Just try to gently remove most of the old soil, then plant in 100% pumice, water thoroughly to rinse off the old soil and fertiliser residues, then put in shade for 2 weeks, then only water sparingly once a week. A cool, shaded green house with high humidity would be very beneficial for recovery. If you don't have a green house, you can make a simple one out of PVC pipes and painters plastic sheets. If the plants are small, like I see of your boxwoods, then put them in a deep 10 gallon bucket, then cover it with a grocery bag. Put the buckets in a cool shady spot. Basically, what you are trying to do is to treat these plants like you would with branch cuttings. You will still lose some more branches in the next few weeks, but hopefully, the plants will recover in time and allow for at least a few viable branches left to work with in the future.
Good luck to you. Please give us some updates.
P.S. The trident maples don't look that bad. They might not need a complete soil replacement at this point, but you should at least remove all of the visible fertiliser granules by taking the plant out of the pot and shake and rinse off the root ball. If they seem to get worse than what you have shown here, then repot completely.