Thanks for this, Billdo - very well-written.
…is I think what you meant 
Been there, done that. Trust me. This is not an easy fix. It’s not about the mess. It’s about the person that created it.
My dad is not nearly that bad. But it’s bad. My Wife was once outside not wearing shoes, and put shoes on to go in the house. I can’t tolerate spending time in there, so it makes it very hard to visit.
Anyway, we rented a 20 yard role-off and filled it in a few days. Couple of months later, it looked like we never did anything.
It’s quite depressing. That and how he has let the place run down. We can’t do everything for him. And for that matter, he really resists any help at all.
And frankly, I can’t help being a little angry. He is going to leave us a terrible mess and a tremendous amount of work. The house needs about $15,000 worth of work. He’s got it, he just won’t do it.