You can grind out the thread on the Break/Nut itself, and epoxy them on, I have done it, and they can be removed easily with heat gun or torch to melt the epoxy if you decide to thread later. It works, and is quick and easy.
As for the thread adapters, I got one years ago, to add a zig zag brake onto a SAR 2, it shot off, using standard screws, so the solution was......
Index the brake on the adapter to work when its tight to the front sight base.
Put it on the barrel, without the three set screws in place, and scribe the location of the screw holes on the barrel.
Drill a small divet on the barrel in the three locations you just marked, you will have to center punch the marks, and use a bit about the size of your screws.
Change the standard flat bottomed set screws to ones with the "V" pointed bottom, that better fits the divet shape you just drilled in the barrel'
Crank tight and use a low strength "Wicking" Lock Tite to secure the screws.
Shoot. I have hundreds of rounds through mine with no issues, and, if you ever decide to thread, the divets will be hidden under a real brake if you do thread in the future.
HTH.