Sorry this is a huge post!
Only read your posts and those quoted within them. You're living with the reality you where warned about. You updated after 13 months (someone bumped it) to say he'd chucked loads and you felt 'it was just he didn't know what to chuck or not, and all his anxiety had gone away.' I advised then he needed focus on his MH regardless. You now say he brought it ALL with him.
One of those statements wasn't correct. (I don't need to know which) One way or another you are seeking approval with your descriptions of him and your choices.
All that time's gone - he could have been closer to acceptance and help by now, rather than now being threatened with losing living with his DD after the event.
As a hoarder with a fair amount of insight into my maladaptive behaviors, I warned it was unlikely he'd 'cured' himself overnight and issues would re emerge.You chose to not hear or see those things that didn't suit what you wanted. Perfectly understandable, but you need to look at why do you do that.
Look at yourself, as much as him, if you want anything other than him just gone.
I'm years on battling this wretched condition that I've learnt a lot about and actively loathe having. At this point I work on it almost every day, yet progress is still hard and slow, and there's no quick fixes.
I'm an organized clean hoarder, removed as a child from a squalor hoard, eventually. I managed my condition for years to stop it impacting on my children the way my parent impacted on mine, yet one Dc (ND) developed it anyway.
If the symptoms of HD aren't managed it becomes a CP issue. The same is true of the symptoms of CFS and many mental and physical health conditions. Parenting round it can be done, many are doing it daily, but you can't have it all.
You need to start looking at yourself as having issues rather than as a victim of others, in order to move forwards, especially to provide your older DD with the consistent home life she needs. Start with help for your own issues, they are the easiest for you to do something about, and will give you more clarity over him.
I can predict that otherwise you'll now be drawn to see any signs of progress 'the friend' makes, as being the latest solution to 'him as the problem.' It won't be. He may do all sorts under 'friend's' lead, as he did previously under yours. All that's being dealt with is symptoms, and is temporary. It doesn't fix very deep issues manifesting in symptoms of 'attachment to stuff' to the detriment of normal living.
You're carrying your own damage,issues and guilt, which is why you flip flop between 'he's wonderful, he's dependable, he's a great dad and step dad, he stands by you, he does tasks for others without complaint, the stuffs only in the shed, the stuff is everywhere, he's awful, he'd be perfect if only he didn't have hoarding disorder or be ND, or follow his mothers lead, display childishness' etc.
He isn't the different man you want him to be, and even if he manages to understand his own issues, you'll still be dealing with (possible) ND, and hoarding tendencies in one way or another in him, whatever the relationship status because he's fathered one of your Dc's.
You've talked about what you need, and I more than understand. But, it's you who's made choices that mean you can't have all the things you believe you need to be happy and a good parent.
You didn't chose a disabling condition, (neither did he) but you know you have to learn how to live with it and it's difficulties regardless. (typed from wheelchair)
He also has a disabling condition that he can't just fix, anymore than you can just fix your CFS if he wanted you to stop having the symptoms of it.
You have a fundamentally decent man in terms of what he will do and will provide for others, with a host of serious issues that may or may not mean you can't live with him. You dislike his parents, who are one of your DC's grandparents. (not unusual) He's not ready to seek help.
But, you feel sure you can't live and parent independently either physically or financially and say you feel trapped because you have now realized that everything you were warned about is true, but he brings all these other things to the relationship that you want and need. The goldfish grows to the size of it's bowl and always wants/needs a bigger one. That's both of you.
Without being too horrible, the chances of you finding another man who wants a mature dependent disabled partner with two children with two different fathers are low. So if you really can't parent independently, you have to look at where you actually are in life and take responsibility for your decisions, rather than saying others not doing that over theirs, are your biggest issue.
I know how harsh I will sound, please believe me it isn't meant that way at all. Wake up, look at what you do have, stop looking for simple solutions to complicated issues. Decide what matters most. As before I wish you luck.