You’re in a very difficult situation, no doubt about that.
Am I right in supposing that your youngest who isn’t biologically yours, but is from what you say most definitely emotionally yours (and you ‘hers’ if you see what I mean) was conceived during your earlier break up with your wife?
What then is the situation with your youngest’s biological father? Do you know who he is? (does your wife?).
Have you asked your wife’s social worker/therapist - or indeed, a family lawyer of any kind - what WOULD happen to your youngest if you were to once again leave your wife? What would happen to her?? What would happen to your wife? Is she actually capable of functioning ‘solo’ to any extent at all (whether using drugs or not)? Or has she reduced to a ‘child’ so to speak needing ‘someone else’ to do ‘everything’ for her?
Given that you are not your youngest’s biological father, have you sought to adopt her formally, so that you ARE her father legally? Would that be contested by her biological parents, or, indeed, by the authorities? (Personally, I would have thought the social workers would have bitten your hand off if you’d offered to adopt the youngest child! Taking a child into foster care or residential care is extremely expensive so they would be overjoyed wouldn’t they if you took financial and legal responsibility for her?)
Can you repeat how old all the children are, as I think there may have been a typo in your post? Is the eldest eleven, is that it?
Do you talk to the older ones to ask how they feel about the entire situation? Children always know that ‘something isn’t right’ and even if you try and protect them they will understand that their mum has mental issues and possibly substance abuse as well. I grew up with a mum with substantial MH issues, and though our dad didn’t try and explain to us, or even mention it, really, it was glaringly obvious to my brother and myself that something was seriously ‘wrong’ with out mother…so children DO know, even if they don’t know the details. Indeed, I would argue it’s really important for you to be as upfront as possible with them up to the point of not frightening them too much.
In respect of staying with your wife, or not, what do YOU want? If YOU could choose (other than to make your wife ‘well’), irrespective of your children, or your wife come to that, what would it be? Do you feel that enough is enough and you just want OUT for a ‘quiet life’?
What drove you to split up last time? Drip drip drip of a life increasingly unbearable, or a sudden crisis that made you scoop up the kids and scarper?? I wouldn’t blame you for either - my father took a HELL of a ‘beating’ from sticking with his highly disturbed wife - he lost ANY chance of marital happiness ‘second time around’…
Part of the reason my father never ended his marriage was the old terrifying issue of ‘well, who else would look after her? What would actually happen to her if I left?’
Does that haunt you as well?
I’m glad that your wife has fessed up re the cocaine - but financially this is very, very dangerous, whatever the street price of cocaine. That it is illegal as well only makes it even riskier. What do the social workers think (do they know? does her GP know? the police?) etc etc…
Sadly ‘self-medicating’ whether with alcohol of drugs (legal or otherwise) is very common for mental health patients - it’s an ‘instant fix’ for their woes that only leads them deeper into the pit of their sad, confused existence…