Roots in the soil, breakdown may take up to two months after the living walnut tree has been removed.
What about my other 2 questions?
You need to do nothing juglone will dissipate in about two months. You can replace the soil, however, you will need to replace about a foot of soil in a fifty foot radius of the tree. You might try to remove all the tree roots, however, many tiny hair roots will still require about two months for juglone to degrade.