There would have to be a heck of rent concession if the Nats have to move into City of Palms. There's an advantage to being near other teams sites that Viera cannot offer, but if the facility was inadequate for the Red Sox, I don't see why it'd be adequate from a baseball sense for the Nats.
I expect this is mostly just playing cities off against each other and that this does not preclude Viera. Without Ft Myers having a better facility (and it does not sound like it is, necessarily), this just seems like looking for the best deal.