Next step is to soak the thread overnight with penatrene.

Can you secure the ring with vice grips?

 

If that doesn't work it's gunna get messy.

 

Yep, gettin messy, time to maybe think about how to re-design the fitting/fixtures to something like a bolt and propper nut set up,,,

 

and perform the full destruction test on the old fixtures.

I had a similar issue, the inner locking bar contains two (2) inserted nuts that are pressed into the bar and then the attachments (sissy bar side rails) are bolted against guard and through to the locking bar.
I was able to undo 3 of the 4 fasteners but one kept turning within the bar with no appreciable length to grasp with multi grips pliers ect, so I ended up removing rear tire, covering all exposed areas under the guard and applied a small (very small) tack weld to nut insert and the locking bar, weld method was MIG as I have very good control over heat and metal deposit.
Immediately after tacking I applied wet cloth to cool down area as quickly as possible so as not to burn paint. It worked no problem, time consuming though.
Hope this helps, Paul