Can I still send a DV letter after I have been sued?

You can get validation through discovery.

what is discovery?

You really should get a lawyer to represent you in court..it sounds like you may not have time to do all the research yourself. Basically, discovery is when the court allows you to send a set of questions to the plaintiff (the CA) this would be the validation, you ask for everything you normally would in a DV letter. When they can't provide the documents, you motion to get the case dismissed with prejudice.