Have you ever heard of bending an Upper receiver?

[Wheelhorse]

Correct - with the reaction rod, the BARREL is out of the torque path. But the same sheering forces will be applied to the PIN AND SLOT whichever end you secure the assembly with.

Don't believe me? Get a 4" diameter length of PVC and cut a hole in it near the end. Slide your forearm into the tube and extend your thumb through the hole. Next have someone hold your exposed hand, which is protruding out the end where your thumb is extended through the hole. Now have them twist the tube until your thumb hurts. Stop. Now have them let go of your hand, and twist the tube again. Stop when your thumb hurts again. That was your elbow and shoulder preventing your thumb from rotating with the tube.

The handshake represents the reaction rod. Your elbow and shoulder represent the barrel clamp. Either way you do it, your thumb gets to feel some pain.

I agree except for one thing. Your test doesn't take into account the lessening of the twisting force on the pin as the barrel nut is tightened. The tighter it gets, the less stress there is between the protruding pin and the slot in the receiver as everything is squeezed together. Remember, we are turning the barrel nut, not the barrel extension and with a little grease or anti seize applied, the rotational force transmitted to the extension is going to be minimal.
I still think that the most direct, with the least number of parts involved way to counteract the force of turning a barrel nut, is with a tool that locks in the lugs of the barrel extension.
Thats my story and I'm stickin to it.