So with your permission your son is already in state C, your wife is going to be joining them , do you really think everything is going to be magically better in state C..? And they both will be returning home?
Just my humble opinion, but the longer the child is with them,the more likely the courts will agree for them to keep him, as he will be in a thriving situation. Also my opinion, sounds to me your wife is planning her escape states away from you, and you are helping make this happen. Oh let me guess she is just doing her additional recovery in another state to be near her child.? Nope not buying it.
I see this as a pay now or pay later situation. But I would certainly want my child with me in the state I reside, pretty sure your state also has additional rehab facilities.
You may have to hire a nanny, you may need to take out a loan to help with costs of childcare, until something more permanent can be found. Maybe you need to take time off work to find the right help.
I am agreeing with you I do not trust what is transpiring in this situation. That is why i am suggesting you get your child home.