Update: This is embarrassing to have to admit, but I had not noticed this in the htaccess file for Server Account B:
RewriteRule ^(/)?$ primarydomain/index.html [L]
I had just assumed that line said .php like at the other account. ::Big Blush::
Thanks again since you always do everything you possibly can to be helpful, and even the BlueHost Support Tech that just helped me discover my oversight is impressed with your fine work!