Fonts are mysterious to me, but I can make a few observations
that may have escaped you. (1) In psnfss2e.pdf (included in
the \doc\latex\psnfss directory distributed with MiKTeX 2.9),
the "Helvetica" is actually NimbusSan! (2) In 00readme.txt
(same directory), in the section "Fonts required for PSNFSS",
it is stated that "Free substitutes for the commercial PostScript
Base fonts are available from the CTAN directory fonts/urw/base35",
and a visit to that directory confirms that NimbusSan is the
'drop-in' replacement for Helvetica.
I am *guessing* that further, more informed reading of those
two documents may point you towards a solution.
Lee Rudolph