Wow. I definitely wouldn't send that to her at all. I would have a face to face discussion with her about the finances and that's it. The rest of that letter is major guilt trip and she most assuredly won't want to speak to you again. That would really push her away and what would you expect to accomplish by telling her all of that? I know you said you won't send it but I really hope you mean that.
I know she is in selfish mode Ed but it seems to me that you need to drop the rope with her. You seem to be in an on going circle with her. You cannot control her let her make her own mistakes and learn from them. I know it hurts you to stand by and see this happen but by detaching/dropping the rope you remove yourself from her actions.
Ed I think your sitch is different than a few of ours because your W fell off the wagon. There's nothing you can do to help her she has to hit rock bottom before she can fix herself and she has so much fixing to do it seems from reading your posts. IMHO there needs to be consequences for her actions especially when it comes to your S. I see her as neglecting him and that has to stop for his sake. I think the focus of your conversation would her should be about your S and not about the two of you.
M 44 W 43 S 23 S 15 INILWY 9/11 Divorce Mediation started 3/13 June 30 the day W is moving out