OP, I have been in a very similar situation with my little girls dad....I could have written some of your earlier posts word for word.
I actually asked for advice on here at the time and received some harsh but true advice, which I didn't act on straight away ("I love him, want to help him, he's a great dad {ha}", etc) but the situation with him became so untenable that he was living separately very shortly afterwards. Removing him from our daily lives was the single best thing I have ever done for my beautiful, bright, confident, intelligent dd. I'm so proud she hasn't had to grow up among drug paraphernalia, erratic mood swings and coming second to his next high. Living with him was confusing, stressful and depressing. My own benchmarking of what was acceptable was totally skewed, as a simple consequence of being around him. I wouldn't have accepted this at the time.
While I felt extremely conflicted about separating him and dd, the reality has turned out to demonstrate his fundamental lack of commitment to her which was always there, demonstrated early on in his low level continuing drug use. He's gone through short periods of seeing her for an hour every week, but these periods are punctuated by long absences. At the present time it's been over two years since he last saw her and he's only about fifteen miles away from us.
I can understand your quandary regarding your DS dad - and your misdirected frustration at his parents. Believe me, they will have seen all this before a hundred times. Addicts can be frighteningly deceitful and plausible and it sounds to me like he's saying all the right things - enough to keep you hopeful and supportive - while showing he intends to do the opposite and continue indulging himself at the expense of everything, including his own child. His parents are probably emotionally spent, and in self protective mode. I'm sure they're also angry and sad that their grandchild hasn't been enough of a motivation for him to sort himself out. As others have said, he's not in recovery - he's not even coming close.
I can understand your desire to help him, really I can. My advice to you is that it is simply not possible while he's still using and that as a mother, you have to consider seeking support for yourself, to ensure your sons safety. Because he isn't safe at the moment, genuinely he isn't. Your partner is the only person who can help himself into recovery, and at the moment he doesn't want to do that.
The best thing you could do for yourself and your son is to get some help yourself - I'm sure now that ss are involved they'll try and facilitate something like this for you, but as someone who is totally anti-drugs I don't know what to say to you in terms of your own use, as it sounds like you're also finding justifying reasons to continue, a bit like he is.
It's genuinely haunting and sad for me to read this thread, but it has also made me so thankful for the decision I made at the time to walk away.
If it would be at all helpful for you to see some previous threads of mine I'll find them and share the links, just say the word.