If this is important you NEED an attorney. It is very unusual for the court to overturn a POA. Why do you want to try? Another avenue would to be get APS involved.
If mom is competent she can change it. A diagnosis of dementia does not mean she cannot change it. It depends. Early stages are still able to make changes, usually.
You need to know the law and there is much to know. There is protocol in the courtroom and in the way the case is run before the judge. I agree with Barb...if the other guy has an attorney you should too. If you have as much evidence as you say, hiring a lawyer should pay for itself so to speak. I was just in jury selection for a tax case and the defendant tried to go solo. It was painful to watch. He had to be coached by everyone in the courtroom (to a point). He lost.
Most attorneys will provide a free consultation at the end of which they will offer a quote for representing you in court. I strongly suggest your present your paper trail and facts to an attorney during a consultation and allow him to give it a review. At the very least, the attorney may make you available of a venerability you didn't consider.
If mom is competent she can change it. A diagnosis of dementia does not mean she cannot change it. It depends. Early stages are still able to make changes, usually.
If yes, then you need one.