jayscott I just read all and my gut said please don't drop your boundaries. It is great that she is pouring things down the drain, throwing cigs away and asking you to call for treatment. But she can still go buy more and if she is serious why is she not calling for treatment. Trust what you see right in front of you and not what you hope will happen.
I cannot even count how many times stbxah promised to stop drinking and drugging. First it was I promise if you marry me I will really stop. Then when we adopted our first son--we almost lost him because although he told me he was not getting high anymore-he failed his urine test. He came home, gathered all his parephenalia together while crying and drove off to throw it away. He was back to drinking and drugging shortly after older DS came home.
Addiction is a horrific beast. I am also the daughter of an alcoholic. No one can stop them but them. It is great you are taking care of your son. You also need to make sure you are taking care of you. You are carrying a heavy load and your son needs one parent that is whole--that is going to be you so make sure you take advantage of all the help people are willing to give you. EAP is great--can they help you with child care options after your MIL leaves?
When your wife's mom arrives--will she take over the care of your son regardless of what your wife/her daughter says so you have some peace of mind?
Note on the breast milk--contact the local hospitals. There are many donor moms who provide milk and you may be able to get breast milk there--especially given the circumstances. They may even give you the name of a donor who can provide you with breast milk. I had a friend who was a donor and while your situation is different than some-she would not have hesitated to provide breast milk for someone in your situation.