Sometimes trees that miss regular watering in pots display this in the foliage. They are dried out or dessicated parts of the leaves and will remain until the tree goes dormant and the leaves fall off. Nothing to be concerned about if it is from missed waterings. I will defer to the experts if they think it something else. John

The only fungal disease known, that affects JM's (and many other trees & woody plants), is Verticillium Wilt. It develops on the roots and blocks the uptake of moisture & nutrients to the foliage. Once it becomes well established, the tree will die. There is no known treatment or cure. Almost any of the JM's you purchase, is already infected with the disease, since propagators don't usually test rootstock before grafting. The one's in Oregon are particularly known for using dirty rootstock. That being said, you can usually prevent the disease from multiplying and seriously affecting the health of your tree. Select a location that is shielded from the sun during the hottest part of the day. Keep the soil moist, not damp or soggy, in the root zone. Do not allow it to dry out. Do not overfertilize the tree. Apply a light application of balanced fertilizer (12-12-12 is fine) in early spring, when new growth begins. Use compost in late summer, for additional nutrition. Apply mulch over the roots, to conserve moisture. Leave an air gap of 3-4 inches around the trunk, to prevent insects and diseases from entering the tree trunk. Do any pruning in early winter, as the sap rises early in Maples. Check any tree before purchase, for signs of disease, such as dead branches, curling or yellow (chlorotic) leaves. The Coral Bark JM ('Sango Kaku') is a good choice. Very colorful in the winter, with it's red bark. Mine is about 20 years old and it receives a "haircut" almost ever winter, to keep it in shape and height limited to around 8ft. It's at the drip edge of a White Oak and a Southern Magnolia.

You forgot to attach the images. You've described the kinds of activities that can certainly kill valuable trees. The planting bed was certainly a major cause of the problem, as were all of the rest of the construction activities. When you remember that those essential tree roots are quite shallow and very wide spread (much farther than the canopy), you'll realize that all of that activity put your tree in jeopardy. Another factor in this situation is the maturity of this tree. Well-established trees do not take this kind of major disturbance well. Destruction of major parts of the root system can be fatal.

Have never encountered Osmocote in a tablet form - that's a new one on me! Generally it comes in a jar with just the loose granules and I apply a small amount (1-2 Tbsp depending on container size) each spring as new growth begins. It lasts pretty much all season although I do supplement with a liquid (Dyna Gro) from time to time during the summer months as it provides a lot of the trace elements/micronutrients that Osmocote omits. I would think one tablet is sufficient for most containers. And I am not sure I really consider the development of dead wood on JM's as 'die back'. Certainly die back can occur from winter cold damage as well as disease issues but the typical dying off of small twigs and stems is a completely natural phenomenon with JM's and nothing really to be concerned about. For whatever reason, many of these smaller stems or twigs just fail to survive and you seem to see it more with the weeping dissectum types so I am guessing it has much to do with congestion and shading. There are other trees that develop a similar trait - Styrax japonica and various birches comes to mind - so this is not unique to JM's.