Although I agree with your assessment that all MERS as nominee style mortgage assignments can be fought in three ways, I have to bring to your attention Florida's 5th District Court of Appeals decision in Taylor v. Deutsche Bank, 44 So.3d 618 (Fla. 5th 2010).
In that decision, the 5th DCA opined that MERS is a nonholder in possession of the instrument who the rights of a holder by virtue of the language in the mortgage document. IMHO, this opinion needs to be overturned. However, for the time being, this is what those in Florida are fighting against. We have 3 out of the DCAs rule in favor of MERS.
Here is a Google Scholar link to the Taylor decision: