Was the tree already there when you added a foot of compost? Hostas like to be in the shade most of the time. Are they in that situation? The compost may still have been a little 'hot' to plant in, in such a thick layer. The first couple pics give me the impression that the soil is very dense, airless, soggy wet all of the time, but pics of 'dirt' can be very deceiving.
Adding a pic of the overall area might help folks understand the situation.
