With the corrosion you are probably looking at replacing the whole valve
I went to replace one of mine & the newer valves did not match up with my existing valve
I would have needed to try to remove parts from the radiator (50's)
I decided it wasn't worth the possible problem of breaking a part off
I do not turn them on/off & mine was not in as bad a condition as yours
Splitting that bonnet nut comes from over tightening it. Due to the packing having worn out.
So on your other ones, that have to be tightened a lot. Replace their packing, and you won't have this problem with them.
You'll be hard pressed to find a bonnet nut for it. New valves aren't made anywhere close to the same. And those nuts where never really made as replacement items.
I recently had the exact same problem and could not find that nut anywhere. Even went to the one plumbing store that carries everything and they said you cannot buy them as individual units.
Ended up buying replacement valves which installed easily.....once I broke loose the old ones. As Scuba said though, you have to be very careful with them as to not break something off inside the radiator.