Gugie, I have a couple Peugeots with DuoPars. They are in the bike shed at the moment but I have never had any issue with the mounting bolt interfering with the chain. I do use the thin nut on the inside of the hanger to lock the derailleur bolt in place.

I would do as you suggested and put a washer in between the bolt head and the derailleur body. Leave just enough bolt on the inside of the hanger for the nut.