You can't put your testimony into the record of a court case with a letter to the judge, because the letter isn't under oath and even if you had it notarized as an affidavit, it can't be cross-examined.
I'm afraid that you're going to have to go to court in Washington State, and I think you're going to need an attorney there who has experience in elder law. The situation you're dealing with here is difficult, but not really unusual, and there isn't anything that you can do other than dig in and start working on getting the truth in front of the court.