Z, I have remained fairly silent on your new threadmostly because I feel others can help you understand things better ESP Anc. I think she can really be good to have in your ear at the moment.
I will just add a few thoughts that I have had though.
Firstly I don't think you need to convince yourself or others when you are finished. I believe that we will know that when it arrives. So maybe you are looking for support and help to continue.If that is true I hope we can help.
Summarising your first post. You are not to blame and she is not to blame for the downfall of your M. Or at least in it's salvation.YYes that does come across that you are accepting an inevitable end. I will just say that this could be also the beginning of letting go that rope. You have been great in your efforts to save M. What you put up with for so long. All the reading, learning and implementing is commendable. Your GAL activities were at least for me inspirational.As you said, you have looked within and made the needed changes. You can be proud of all of that and have no shame if M ends.
As you are now digging deeper into W's issues, which shows you are not done, you may be focusing too much on her side if the fence. It is important to understand the situation and I am definitely not saying it is wrong. But just keep some focus and energy on you.
Last point. You have been forever present as a strong rock of love, that has always been there for your W. If you do get to the point of having enough, I'd ask you to consider removing this from your W first. If she feels she can never lose you, she may never have the motivation to really try. After years of effort you could be inclined (or pushed by a moment of frustration) to just make a quick clean break. I hope that you understand thus us meant as friendly advice and not judging you or your thinking.
Happy thoughts my friend
R 25 years M 14 years S11 & S13 Working on it alone since Oct 2014 M in trouble a lot earlier (~2 years) Feb 2016. 1st R chat in a yr. Next R chat Aug'17 Still together