Your poor girls are already trying to fix their parent but that parent is you, not him. They have given up on him already.
They see YOU feeling ill and tired and stressed and they don't know how to make it better.
Do you not realise that this is also what makes them upset and tearful and angry and unsettled and awful?
I understand that you think you are protecting them by sticking with the status quo.
You aren't. I promise you.
Blaming yourself and bigging up your husband whilst simultaneously saying you are splitting is putting you right back at the point of enabling his behaviour and pulling the rug from under them. You are the nearest thing to stability they have and now you are going to lie to their faces and tell them that you are the bad one?? Seriously??
They will already see their father as a negative force in the house. He treats one better than the other? And you let that happen? And your way of dealing with it is to toughen up the older one? How the hell is she ever going to feel selfworth and know what a good (normal) relationship is like if her own mother allows her to be treated like that and her own father is then put on a pedestal?? So confusing and so so damaging.
He isn't a bit unwell. He is an addict. Addiction = selfish and addiction is a dirty, horrible, lonely and self-absorbed place to be. You can't change that. Your poor kids will already know that they can never be good enough for him and neither will you. Do you realise that?? No amount of smoothing over the cracks will change that.
Yes, addiction is an illness of sorts but there are ways of explaining that to your children without either bigging him up or slagging him off or leading them to think you or they are responsible for his actions.
Where does he smoke these joints? At home? When the kids are in bed? In the garden? In the living room? Seriously??
You are very dismissive of al-anon. My experience has been very different to yours and I think perhaps you are hiding behind all this 'not falling over' 'not abusive' stuff because it assuages your own guilt about your inertia.
Stop playing the martyr and start playing the mother.
Have you read the Al Anon 3 act play?? It isn't as long as it sounds but it resonated so well with me and the rest of the family. Here is a link to one version:
www.soberrecovery.com/forums/friends-family-alcoholics/137214-alcoholism-merry-go-round-named-denial.html