Legal Question in Real Estate Law in California

Yes, but you will have to have the court issue the subpoenas. One of the few real-world effects of the largely ceremonial title "officer of the court" that is bestowed on attorneys is that they can issue their own subpoenas. You cannot.

Any relevant records "reasonably calculated to lead to the discovery of admissible evidence."

None.

I disagree with Mr. McCormick. If you actually sue your lender, they are a party to the lawsuit and a subpoena is not used. A subpoena is used for third parties who are not parties to the lawsuit.

If they are a party, and you want documents, you have to comply with the Discovery Act and propound inspection demands for documents.

Complying with the Discovery Act (in the Code of Civil Procedure, sections 2016 et seq.) will be simpler for you (compared with use of subpoenas) as a non-attorney plaintiff, but by no means simple. You should consider getting (at least) an attorney or paralegal to act as an advisor. Mortgage lenders are very tough on suits brought by former borrowers, have lots of experience, and will use stautory and case law that can be quite non-transparent and confusing, even for attorneys.