MCE..….from what you share, I see the situation, the same way that you do....
Sounds like your wife is being the central "client' or "patient"...…
To me, it sounds like State M is the location that would be best for the child....for a number of reasons.
The child should be the central person, as I see it.
It seems like you can provide the most stable environment for the child. Being the most able to provide economic stability and geographic convenience...and, believe, with children, geographic convenience becomes a big issue.....
I get it, that your wife's family would LIKE to have their family all in the same location....but, the total picture needs to be considered. Your wife's plans are centering around her needs, right now. Which is fine, for her. I do see where her sobriety needs to be the number one consideration, for her. Without sobriety....she will lose everything. But, after her sober living time is over....she will have to face life on life's terms....
I understand that this must get on your last nerve....but, I think you do need to stick to your own boundaries, on this....
After all, who knows if your wife will be content, in State C, after the novelty wears off....
Remember the "golden rule".....Say what you mean; mean what you say; but, don't say it mean.