Myself and MarkG were just discussing this last night strangely enough and here we are. I have recently sheared two indexing pins on barrels when using the Reaction Rod and honestly it could have happened with a clam-shell as well IMO.
The best option here (unless you have a pinned muzzle device or some other prohibitive nonsense) is to simply cut off the barrel nut and then replace it. If you shear the indexing pin that barrel will more or less be toast unless you find someone that knows what they are doing and they are good at it.
When the nut "seizes" to the upper, the upper and nut become one. If you apply force to it the upper will flex. I have even seen clam-shells get warped from this happening over and over. This is why I always tell people to use anti-seize and if they insist on using a torque wrench set it to the lowest setting (30 ft./lbs) and then align it from that point.
If Rocksett was used you will need to use hot water to loosen it. I have done it with muzzle devices and it worked. Heat won't because the Rocksett is designed to be heat resistant. I also put my stuff in the vise in such a way that I am applying downward force to the nut rather then pulling up.

