Stig
To my mind the problem is caused by the short adapter you're using between mirror stem and the handle bracket. Specially the shiny adapter (bottom, right on the picture - with the short male thread) is not suitable for an application like this . The threaded part on that adapter is WAY too short, and doesn't penetrate the handle bar bracket sufficiently to ensure a firm grip at the threaded part of the handle bar.
To be honest, I don't think any washers or Locktite products will solve your problem - rather get adapters with longer threaded parts, or make up an adapter - and you should be fine. If the threaded parts on an adapter is long or deep enough, there should be no need for any lock washers, or Locktite products.
I also did an almost similar modification some time ago, and just used long 8mm Nuts which are threaded right through. Ad the bottom of the nuts I just used a longish piece of 8mm rod, applied some red Loctite on the one end - and then screwed that part of the rod quite deep into the long nut. I then just left it for a while, and afterwards just screwed the threaded part of the new adapters normally into the handle bars - without using any Locktite or lock washer.
Then I just screwed the mirror stems into the long nuts, and tightened the locking nuts on the mirror stems normally - without using any washers or Loctite. So far the mirrors took a couple of hard knocks on bumpy roads, and everything is still rock solid.
A picture of my modification can be seen below.
