@WVV,
This is Will’s private financial information and it may not be the best idea for you to be sharing it publicly in this way. You seem determined to do what you think is best for Will and I believe that you have good intentions and love in your heart. It sounds like you are stuck in a messy situation where it feels like your autonomy and Will's are being steamrolled by an over-involved mother.
I agree with @AuroraBorealis that speaking to your father and getting more advice from him could be a useful place to start. All of us replying to you now are strangers to your situation and we do not have all of the pieces of the puzzle. I'm concerned that engaging in secrecy and going to battle with Will's mother has a high chance of further damaging your relationship with Will and his mother and possibly getting you into trouble.
If you believe in Will's independence as much as you say you do, then it will have to be he who deals with his mother - it is not your role to "save him" from her. It is evident that you and Will each have a parent in your life who is concerned about your well being and involved in big life decisions that you want to make. If you would like to change this dynamic, I suggest relying on your therapist and your father to determine an appropriate course of action.
It sounds like you are working on some big and important goals like saving your money to get your own place. I think this is wonderful and striving for independence and trying to help someone you care about are very good things. That said, family relationships and conservatorships are very complicated and it is important to be consulting with people who have a better understanding of your whole situation than any of us here on the forum could. Afterall, we just met you. Maybe you could arrange a face to face meeting with you, Will, his mother, and your father.
