FatherOf3, I appreciate how you seem to be looking at all the angles. Perhaps she's as unstable as can be, or perhaps she's just situationally depressed. But here are my thoughts.
You've mentioned how hard she tried to fit in with your family. This suggests that you view your first family as the anchor, and I understand that. But this baby, to your wife, isn't merely a bonus round - this IS primary - her central and only focus. Not to keep trying to make it work with a group that's been in place for years already. Not that you coerced her into it, but it sounds exhausting for her to keep "fitting in" to something already there, something that is clearly your main priority. Becoming a mother to her own child of course is far different than being a stepmother to your children. I completely get that you wouldn't want to be away from your kids just because they're grown up. But it must make her feel like an outsider all the time. I wouldn't want to raise my child like that either - with everything already set up by you years ago in your former personal life - now she must live within those constraints. It just sounds so stifling.
Your posts are full of logic. What you have proposed to her - staying in your current city for 3 years and then moving on - is very logical. But those first three years - pregnancy, babyhood, all those "firsts" - may be the very years she needs the security of the place where her family is. It sounds like she needs that security right now, not logic and grids and timelines. She may feel the need to nest in a place that's HERS - not yours, just something she's latching onto. Of course, logically, she should have explained exactly what she would do given any situation. But how do we know, in life, until it happens? It's not a logically drawn chart - how we feel often guides us. It makes sense to me that her constant priority will not be your grown children.
I'm certainly glad you've dropped the notion of abandoning your child. There would have been no other word for it, regardless of all the thought behind it. Better for the baby to know you're available somewhere, and constantly interested and keeping in touch - than disappearing altogether. That seems unfathomable to me.
Maybe your wife, with some time apart, WILL feel differently. Maybe it's the idea of having to live this life that's already set up to suit your original family's needs, that makes her want to be away from it. I too would be very uncomfortable with the idea of being a permanent guest in someone else's life. (I know you never worded it that way, or even thought about it that way, and it sounds like you've made many efforts to accomodate her - but at the end of the day, it's your life plan, not hers.) I would go with the plan you mentioned of no contact for a bit - just let her be. Well - I would check in on her to let her know you're concerned about her and the baby, physically. But I wouldn't bring up any huge issues for a bit.
If worse comes to worse and she stays in Atlanta, if I'm not mistaken, you've indicated that you are financially comfortable. If you are not willing to leave your current city, would it not be possible to have a second small home to stay in, in Atlanta? I know you don't want to be away from your grown children but certainly they themselves are doing their own thing enough (or will be) so that you can leave for days at a time. Your youngest child will need you the most, not your older ones. Your oldest ones (doesn't one have a child of her own?) certainly ought to understand that, and can be mobile in visiting. This way, your youngest child's life wouldn't have to be rent in two - he could have the security of his mom, but also know he could see his dad frequenty. It might not be what you planned, but like most of us, one has to make the best of the situation at hand!