Thank you for the opportunity to assist you.
Q) I want to add discovery documents that my attorney forgot to include
If you mean divulge additional information to the other party, then just send the documents to the other party's attorney and file a copy with the court. Make sure to include a certificate of service and sign it with your name and address. Sign it "pro se", and make it clear that you now represent yourself.
If you mean you want to request additional discovery from the other party, you can do that if it's for clarification of previously disclosed evidence. If you're filing all new discovery requests, the judge may or may not allow that, depending on how close your trial date is.
Q) I want to amend the original complaint to change it to adultery
While the rules of procedure allow amendments to a complaint to be "liberally granted in furtherance of the ends of justice," you might not be allowed to do this if it's very late in the litigation. In other words, if changing your theory of the case goes from abandonment to adultery, that might cause all sorts of new discovery to have to be done. A judge may not grant that amendment.
In order to amend a complaint, simply file an amended petition for divorce. It's probably good to attach a copy of the original complaint, too. Make sure you file the original with the court, and include a certificate of service to show you've served a copy on the husband's attorney.
Q) I want to add a witness to be subpoenaed for the trial (the paramour.)
This person probably wouldn't be relevant to abandonment. So, while you could subpoena the paramour, she may not be allowed to testify unless the judge grants your motion to amend the complaint.
Also, you're probably going to run into a 5th amendment problem. While nobody gets prosecuted for adultery, it's still on the books in Virgina. That means that a witness can plead the 5th and assert their constitutional right to silence in a deposition or at trial when asked questions about adultery. The paramour may do that, assuming she knows to. Your husband's attorney may tell her to take the 5th, too.
Attached is a form you can use for witness subpoena, in case you decide to go this route:
Q) My goal is to push my husband to a settlement that is fair-by exposing his indiscretions - this may be most effective
Careful here. You might be setting yourself up for sanctions and for an award of attorney's fees
if your husband's attorney can prove that you are amending the complaint and subpoenaing various witnesses in order to "harass or to cause unnecessary delay or needless[ly] increase ... the cost of litigation.
I understand where you're coming from, but tread carefully. You don't want to be paying your husband's attorney, right?!?!?
I hope my response has been helpful. If you have follow-up questions or concerns on this topic, please ask. Otherwise, please rate my answer positively so that I can receive credit for my work. Doing so will NOT cost you any additional fee.