A leica anti newton rings glass (rare) will take care of the rarely occuring newton rings. It's not a big deal, Imo.

The filter being under the lens is not a problem. Dust doesn't show unless you really decide to make it a bread crumbs holder or a bird feeder. That's a myth. I know, I use a focomat Ic on a daily basis and my filter is dusty under the enlarger's light and my prints are beautiful.