I will try to best answer your question.
You can file objections to the interrogatories and request for production documents on the basis that they are overly broad, burdensome and intentionally invasive. The information sought is also not recent, probative or relevant.
In addition to the objections, a second backup strategy would be to use the discovery packet sent to you as a template to modify and send the same discovery questions back to your ex. You will need to modify so that they are directed toward your ex as opposed to you. One strategy would be you both end up withdrawing your discovery requests and neither responds or both discovery requests get reduced to something more reasonable and relevant.
In order to authenticate evidence you need to lay a foundation. The foundation is laid by a witness who can testify as to what the document purports to be and why it is relevant to the issue before the court. Ie this is an email that I received from my ex on January 15th 2013 discussing ...
The same basic foundation would need to be laid for text messages. If you intend to present any criminal records you will need to have certified copies of criminal records from the court.
Based upon the fact you stated I don't see how your ex has any legitimate basis to contact you or either of your children. At this stage he is a stranger even to his biological child. There is no basis for him to have any contact whatsoever. I think that is your strongest point and clearance undisputed point. I would focus on this point as this case is not about custody or any other issues of dissolving a relationship.