Hi, i'm so sorry that you and your wife and your child have been put through the mill like this.
My first thought on reading was that your wife started drinking soon after the birth. I wonder if at this time your wife was suffering from post natal depression and that this is what caused her to go off the rails and if thats the case has that side issue been dealt with?
Who knows why she told you about this episode, there could be a number or reasons from her wanting to really make a fresh start and admiting her harm and wanting to make amends, through to not being able to handle the guilt and dumping some of it on you or simply her needing to test your loyalty to her becuase she feels insecure and unlovable, what test could be bigger than this. She obviously isn't a bad person at heart or i don't think she'd have signed the pre nup thing. A woman frets over security and I believe she is showing you how much she trusts you by doing this. She obviously wants to gain your trust in return by providing you with proof of her boundry.
I suppose whatever the reason the fact of it is that your wife has betrayed your trust and has left you feeling shell shocked and unsure of where to go with this next. Whatever the reason for this, drink, post natal depression or perhaps she really was inclined to do this thing anyway. I think you should honour her recovery from alcohol but also honour yourself aswell. Take some time out and ask yourself if deep down you could truly forgive and forget and hold no resentment whatsoever, and never let the issue cloud your thinking or behaviour towards her ever. If you couldnt do that, how is it likely to affect you from here on in and also how would that be likely to affect your wife from here on in. and might it affect your wife enough to cause a relapse in her recovery which would start that old merry go round off again. Im also sure that you are thinking about your child as well in all this. perhaps it might be easier not to think about what is best for you and her but what is truly best for your child instead?
Difficult situation I wouldn't like to be in your shoes right now, might be worth asking your own higher power to provide a solution for the best possible outcome for all concerned, and trust that HP will do that.. sorry i couldn't be of more help.