You might get the clearance you need by fitting the nut and washers. The nut is lock nut because you don't torque down the bolt all the way (at least on mine).

My Duopar has a thick felt washer and a thin plastic washer between the derailleur and the hanger so it can swing forward and back. This swing is to facilitate wheel removal. There is a locknut on the inside of the hanger for locking the derailleur bolt with just enough play to allow it to swing. There are little to no threads showing past the nut on mine.

Your hanger might be different if the nut interferes.