They can't outright refuse to handle DMCA requests, but they can stop automated handling of such requests. If I understand correctly, big content holders get special tools they can use to remove infringing content from some Google services (YouTube at least; not sure about search results). If they abuse that special access, Google can take away these privileges and double check everything manually. That takes a lot of time, and means that the infringing content will remain accessible for a longer time.

I doubt that is legal. That (or something along these lines) would be a good improvement to DMCA process in general, but doubtfully something that Google can decide to do.

or maybe charging, if such a request is sent they have to pay for the employee time wasted on that