Unfortunately, the defendants are correct here. You will need to hire a lawyer to pursue this claim on behalf of your mother.
CCP Section 1297.351 does not apply to civil cases in court. I can see how CCP sections 372-76 may be a bit confusing, but they simply mean that you can "stand in" as the plaintiff when you are the conservator. As such, they allow you to make decisions on your mother's behalf in the litigation. However, those statutes don't allow you to act in pro per on your mother's behalf.
You need to obtain counsel. Your failure to do so may result in the court striking your complaint. The case law is pretty clear on this point. See, e.g., City of Downey v. Johnson, 263 Cal.App.2d 775 (1968).