Dear Susie,  Everything you mention in reducing endless screw
friction is important.  However, the greatest frictional losses are
in the sliding between worm gear and worm.  As you suspected, polishing
the underside of the worm thread land is very important.

In addition, though you polished the endstone, you did not mention
checking for a flat on the end of the upper endless pivot.  It should
be a perfect dome with no discernible flat spot.

I would be happy to take a look at it if you wanted to send it, but
start with these two suggestions as the most likely sources of
excessive wear.

Jonathan Herz
http://herzmusicbox.com/