I would consult with an attorney to determine legal options for an adequate defense and expected expenses. A consult is not a contract and you can change your mind if you are unwilling to take the risk with a law suit.

I suspect the take down notice is a DMCA take down based upon an flawed assumption of the law. The hard part about this is arguing the technical merits of the case before non-technical people. While the take down notice is probably in error they could still make a good argument around bypassing their security controls.

You could appeal to the EFF or ACLU. If they are willing to take your case it will be pro bono.