Condolences on the death in your family!
The organic amendments will decompose over time. Under some circumstances, that might cause the root ball and soil surface to subside. I guess if roots fill that zone before the roganics decompose, the roots might fill the empty space left behind.
I think it sounded like you were going to make the transition from "amended hole" to "native soil" at least somewhat gradual. I think that is important.
Even more important is that anything you dig below grade NEEDS a drainage path for water to escape.
Whatever you dig, before you back-fill with amended soil, imagine a 6" water main gushing a Niagara into that hole. Where will the water go? It can't drain DOWN through the clay fast enough to keep your trees alive. And water is stubborn - it WON'T flow uphill. So it will puddle behind any ridge or rise of clay. That's how they make reservoirs. But you want an aerobic root zone, not trapped water turning your amended planting hole into an anaerobic mud wallow.
So there needs to be a trench with a gradual down-slope draining the deepest part of your planting hole DOWN to some lower spot.
