No guarantee, but I think by cutting that stump off to the correct length of the collar, the tree will be able to 'heal' the damage all by itself. More air, less water, less bacterial/fungal life. Plants are very conscientious about damage. They don't want to die just because of a scratch or broken limb/leaf...they have their own plans in place to take care of their health.
As long as that tree is healthy it should be able to deal with the rot now that there is air flow and less moisture.
I'd like to see a thin layer of soil covering the crown of the roots. Only up to the line where bark begins, no higher. You might have to change your walkway a bit but that tree is worth any work and expertise to learn to manage this gorgeous specimen. We could talk about ripping the concrete walkway out, replacing with pavers made of concrete; 2 inches thick set on a sand base worked around the base of this tree. Patio/walkway?
Think about a low voltage landscape spot light up lighting this tree at night...
I would leave this stump alone. Maybe get a hair dryer and try to dry it out a bit? Remove some of the tiny branches of leaves right at the main branch...open the canopy up somewhat to allow more air movement and light as well as great peekaboo glimpses of that beautiful structure. I always went under the canopy to prune. Leaves on little branches that hardly ever see the sun are a good thing to remove. When you remove them remember that you've now allowed other leaves more light access.
What did your arborist think of the cut? The little bit of rot? Very cool you hired an arborist, this tree deserves the expertise. Beautiful Japanese Maple. You do know this tree will never get any higher, really. 8 feet is huge for this little grafted tree!
It would have been better to have cut that stump off at a bit of an angle so water would slide off and away. I can't tell by the pictures. This tree should be just fine. The rot might have gotten into the tree but with more air your tree is able to 'cauterize' any access to the important cambium just below the bark. The interior of your tree will heal with more air. Do not stuff anything into that hole. The collar looks like the proper thickness and the wood looks healthy other than that center.
At 45 years of age or so this tree likes its location and will probably outlive us all. I'd love to help do a little pruning this spring with you on this tree...?