Are you sure you can't back those 2 allen bolts out far enough/all the way out to the point you could get that side plate off or back far enough to just lift it right off from the top?
They had to assemble it originally and I can't think of a way you could put allen bolts in to where you couldn't get them back out.
I have never owned a Nikon M-223 mount before so I don't know for sure. But all the others that I have owned in the same price range and even lower, you could get them off.
Now if it doesn't use the piece where the allen bolts go through to clamp onto the rail, I am at a loss. How does it lock onto the rail?
I am no longer participating in gun forums.