Ice - I think that the WAS knows that we are not going to shut the door on them. Based upon statements from my H, he is pretty confident that he can return whenever (it is part of their fantasy land). I figured that I will wait until he is asking to come back to discuss what will need to change in order to make our M work. It is way too early to discuss that now. Your W has no idea what she wants, except she knows that she needs space.
WAS dont respond to letters or words or promises. They are looking for actions, and honestly, they dont even believe your actions. My H thinks that I am changing to win him back. I dont care if he believes me at this point, because I feel better about myself and that is all that matters.
Instead of writing a letter, you can show your W that you are not going to shut the door on her instead of telling her. You let her know that by being kind and friendly. If she moves out, you refrain from being angry and bitter. Your W will see that the door is open for as long as you decide to leave it open.
You need to let go of any timeline as well. You mentioned that you were preparing yourself for this spring. This journey is going to take a lot longer than you think. You are just at the beginning (as am I). At the beginning, I thought that we would be one happy family again by the holidays which is totally not going to happen. It totally stinks, but you really need to drop expectations to protect yourself. Time is your friend.