I don't have the setup to build emacs on Windows. So I tried applying your patch just to <emacs extracted dir>/share/emacs/25.0.93/lisp/faces.el, recompiling faces.elc and restarting emacs. But the variable-pitch is still "Sans Serif" family instead of being set to Arial.
On my windows, C-h v system-type gives windows-nt. So I replaced w32 with windows-nt, recompiled faces.elc, restarted emacs, and still nothing happened.
I am not sure I any of my previous emails were received, but FWIW, I cannot figure out if edits to that faces.el are being effective on Windows, or if that patch is working for me.