Added a safety feature for when the robot finds itself stuck in the red (surrounded by non-passable blocks).  Also, more importantly, increased the tolerance for passable blocks by lowering the cutoff from 0.3 to 0.2.  This second fix allowed the robot to successfully naviage big frustration world!
narrowed buffer, increased tolerance for uncertainty even more
Played with numbers some more.  The heat map is still a little sketchy, but it's much better than it was and, more importantly, IT WORKS!