So...update. I'm back at home. I feel like maybe I'm overreacting and this is really just growing pains from starting a blended family. Maybe I'm too sensitive to the things that are happening in the house and I just don't know how to cope with the troubles and problems.
I would recommend you keep a journal of how you're feeling every day. How does she act around your children? How do your children seem to do around her? How are YOU behaving as a human being who does not want to be an unhinged, angry, "drama" person. How much joy are you getting from your relationship? Read over it in a couple weeks to a month.
I usually love a happy ending but I honestly don't have a good feeling about this at all, for a few reasons:
1) The way she loudly proclaimed her anger at your young son over a cleaning job within earshot of him shows amazing immaturity and inconsideration and lack of empathy on her part.
2) Hasn't Your relationship already been on the rocks for awhile with talk of divorce/separation? Has she been able to change long term before? She is who she is.
3) You become a lesser person when in her orbit, due to your frustration. Yelling and acting in a way you are ashamed of. Other women have never brought this side out in you. Other women have never acted unhinged in the way she has.
Personally, I think you've been UNDER reacting to the reality of the situation. When I read your very first post I was ready to take her side because I've been a step mother and I know how hard the blended family dynamic is. But when you described her loud criticism of a NINE year old's cleaning job, I knew she had problems and I believe she will always be an issue.
I hope you are prepared to walk the VERY FIRST TIME one of you slips into yelling, screaming, tantrum mode with the other, or as soon as you see she is cold and indifferent to your children.
I'm not saying this to come down on you, just to let you know as an outside observer, I am very concerned for you. Please keep us posted and let us know if you have troubles. Most criticism you get here is not judgmental in that it is coming from people who know they keep making their own mistakes in relationships, and understand how hard it is to see clearly when you're in the thick of things, that's why most of us are here.