My opinion is that the first problem was having it in too big a pot for the roots to have populated the substrate. Being a shallow pot, the roots then tend toward anoxia (drowining). In winter and cool spring there is very little metabolic activity (life!) to execute the processes that go with death, so everything looks to be okay. Then the temperature goes up, metabolism picks up, and the foliage quickly acquires that deathly grey cast. Then the tips die and the cascade of the foliage drying a crumbling begins. As everybody has said, it happened a long time ago.
When you try again, use a deeper pot. That will help a lot, based on my experience. I also suggest following the axiom cramming it into the smallest pot into which you can shoehorn it - but that is for later on.
To these suggestions that the roots got cooked by where you kept it - maybe so. BUY a simple MEAT THERMOMETER probe from the grocery (less than $15) and stick it in the substrate. See what the temperatures your tree's roots are really experiencing instead of 'imagining' what they might be. Keep the roots below 95F. Do whatever it takes to make it happen.