I think I'd like (b) and (c) to be at the will of the wearer. It just seems like more of an enticement to put the thing on.
I don't like the idea of making the transformation into manscorpions optional - the original text says the crown "has no real practical uses, instead bearing a terrible curse". The enticement was that Shattados thought the crown would allow him to "bend others to his will". Which it did, sort of, since his master scorpion powers allowed him to control the manscorpions the crown had created.
Anyway, that's probably enough for me. Do you think it needs any more properties?
It turns the surrounding land barren, although it's unclear how quickly this happens. If the crown is destroyed the Bright Desert "slowly begins to revert to a reasonably fertile, if somewhat arid, region. These changes take place over a century or more, and will not affect ongoing campaigns."
So it might have taken a century for the desert to become so lifeless around the Shattados's palace.
Actually, that makes me realize something - I'm wondering whether the Immortality also applies to the scorpion creature created by the crown. Consider these quotes:
In an agonizing moment, he was transformed into a monstrous scorpion and his people into the wild manscorpions which still plague the desert.
These savage, inhuman creatures were created centuries ago as the result of a curse which wiped out their thriving empire. Once men, the manscorpions' only remaining joy is to seek vengeance against all those normal beings who do not suffer as they do.
Those suggest the manscorpions might be the same individuals as were transformed when Shattados put on the crown, despite that being 2,000 years ago.
A Manscorpion's maximum age is 80+2d20 years according to the New Character Races information in the Orc's Head Peninsula
sourcebook. That's for Nimmurian Manscorpion though, who I guess might have different lifespans than other breeds of these creatures.
However, there is also a line that "Its ruins are ravaged by the descendants of Shattados
" which might indicate the Manscorpions the curse created are the ancestors of the ones currently in the Bright Desert.
If the Manscorpions and Monarch Scorpions the crown creates are immortal it would help explain why (a) they all die if the crown is destroyed and (b) they can survive in such a barren desert - maybe their immortality means they don't have to eat, and they just prey on animals and humanoids out of their lust for killing?