I bought a D46 quite a few years ago that had obviously seen a fair amount of use and the underlever lock up that you refer to had become a little weak (not a great design by Diana).
Occasionally the underlever would drop out following firing and I subsequently purchased a strong rare earth mini magnet from a certain auction site and used a two part epoxy glue to attach this to the barrel. It works very effectively and the lever never drops out.
I'm really unsure that this arrangement adds anything to the accuracy but doubt it. It does make for a far more positive lock up however.
Dave