I think your letter sounds really good so far. I hope you and your t don't just 'agree to disagree' on this because it sounds like it's important that she understands your point of view and is on your side. It doesn't sound to me like something 'best just left alone'. To me it makes sense that depending on someone for survival doesn't automatically equal trust. Maybe your t has never really properly thought about it before - I think your letter might give her something to think about.