I was going to argue with you until I realized that your Blight Counter change would work ridiculously well.
To deal with Rats, in addition to those changes that you mentioned, I'd tack on the requirement for Hamelin to discard a card when he summons a rat from enemy models.
Meaning the Rats retain their core power and mobility, but in order to overwhelm his foes, he must deplete his hand.
Keeping the Void in mind, I believe that this would be a very unique interaction within the crew.
The only argument I would make against your changes, is that (All)Abandon should still apply insignificant.
Simply because it gives a dichotomy to the Stolen outside of "Stand there so if I f*ck up Hamelin comes back."
But other than that, I believe reducing the range on Nihilism, changing the bully mechanic, and reducing the amount of methods Hamelin has to instaban targeting, you're on to something.
Well done, +1 for you.
EDIT: Oh, and I also believe that Rats in range of Nihilism should be capable of becoming Significant.