To bad about your experiences with Mamiya. The 645 system it seems has always been a pimple for them. I would have thought they would have been a little more lienant on the charges, but then again I don't know all the cirumstances.
It's kind of weird, the 330TLR and the 7 have a reputation of having sharp lenses, but the 645 has alway been thought to have soft lenses. I wonder if they are farming out their lens production and the 645 lenses are being made at some inferior operation.
I started out with a Minolta SRT 101 (actually really started out with a borrowed Rollie TLR) and I loved the camera, but the lenses were never sharp enough for my needs. Switched to Nikon and never looked back. For 35mm that is. Do mainly MF and LF now.
My ramblings don't seem to address your issues but what I am leading up to is maybe you should ditch the Mamiya and go to the Pentax or Contax 645's if you like that format. I understand the optics for both of these systems are very good. I have seen large enlargements made from the Pentax and they were very sharp. If the Contax uses Zeiss glass then your ok with that one. The lenses on my Blad are great.
I used a Bronica S2A for years (with Nikkor lenses) and one of the backs I had was a 645. I finally found that I ditched in favor of the sq 6x6 back. Just didn't have to mess around when I wanted verticals, especially when using a flash.
You could also check out the new Blad 645 as well, but I hear it uses Fuji glass.