I'm sort of on the fence about the idea of drawing a line in the sand with a letter that may come back to haunt you later for reasons you can't identify at this point in time. Then again, I like the idea of a Disfellowshipped person"owning it" and not slinking away into the wild blue yonder without having his or her say.
If you'd care to, I for one would be interested in seeing your letter before you send it. There's a lot of clever people on this forum who will likely have valuable input to help you through this life changing move you're about to make.
Either way, you know your situation best and it sounds to me like you're sort of relieved to have a plan.
Just curious...did your Mom bring up the topic of "reinstatement then fading" coincidentally or does she know that was your intended plan at first? I can't imagine you'd have told her that was what you were thinking of doing.