I have surrendered
No more buying chemicals (suggested and not suggested), no more pushing and pulling bore brushes (wore out 11 of them), no more sleep deprivation due to worry about a fouled barrel, I have recognized a point of diminishing returns and surrendered to a superior force.
Even soaked in Naval Jelly for two days. Beyond a little black residue, no mystery fouling has been removed. Reached a break even point 3 days ago signaled by almost no black residue. I have no idea what has fouled about 7 inches of this barrel. Whatever it is, it is refractory and impervious to all removal efforts. Must be an unknown space age material.
Thanks for all comments and suggestions. A replacement barrel will cost about the same as my investment in cleaning supplies.