What a lovely non-answer. The rules of the site don't dictate Proteus has complete control of the Gem. In this thread he possesses it.
The Gem as an artifact is powerful, very powerful, in the hands of someone who knows how to use it properly, and knows how to use it properly in absence of the other Infinity Gems.
Proteus with no prior knowledge of the Gem, or how to use it, or how to use it in absence of the other Gems, and relatively dim, does what against the reality warper who remade him as she unmakes him as casually as she did her other creations back into nothingness - as casually as she did her reality of her own creation?