I know nothing about these trees, but since the first order of business is to get the roots into some media that will allow you to enter the cycle of grow, trim top, trim roots, ad infinitum, and since you have not been successful in getting the roots free of the mother soil in any other way, I'd try to only wash away the clay. It's not easy in any instance and can't be done at one sitting, but might work.
Put the whole ball in a vessel that can be filled with water above the rootball. Every day hose off what will come off and put it back in the vessel overnight. Don't use anything to comb the roots, just wash soil off and keep the ball wet. Don't use a pressure washer, just a garden hose with a tight stream. Almost any tree can stand being soaked for a few days and full sun all day will help. Some roots are fleshy and will take this poorly, but piney roots are usually very fibrous and stringy and tough and should wash just fine. If these roots are the piney type do the whole job at one sitting. If all the clay is off in one day, decide how much root you need to keep and pot up appropriately. I'd err on the side of keeping too many because you're new to this species. If it takes days to do this, or if the roots are fleshy I'd keep them all.
Nothing is ever as easy as it sounds, so expect the worst and just persevere and maybe you'll get lucky.
