"As a mother I just can't imagine getting to a point where I break ties with my son. I'd always keep supporting him and being there for him. Aibu to judge them?"
Sorry, but you are being massively unreasonable. And, I suspect, you are minimising the damage wrought by your husband. Maybe it's the only way you can cope with it, maybe you're naive - I don't know. But please, whatever you do, do NOT say this to them, particularly the 'as a mother'. It makes you sound very holier-than-thou, and frankly a bit of a cunt. Also - sometimes the best support you can offer someone is to break ties, because to maintain them is to enable the addict. And if you can't imagine that, the fault is with your imagination and not with your in-laws.
"I do feel guilty about situations my ds has been in and I'm no kind on amazing mother but I can't see my mil's stance as anything but spiteful now he's recovering."
Again, YAB massively unreasonable here, and assuming spite where there is none. And I think you should have a think about hypocrisy. You are condemning her for not putting her son before her own mental health and welfare, but you are fully prepared to put your husband before your son's welfare. You've put your son in situations bad enough for you to feel guilt about it. You have NO RIGHT to judge her!
And if your in-laws have chosen to cut contact with you and DS too, maybe it's because you make it clear you expect them to give, give, give of themselves with no thought to their own welfare, while you and your husband take, take, take and put your son into situations that even you feel guilty about.
Sorry to be so harsh OP, but your minimising of the harm your husband has caused suggests to me you need bluntness to get through your veil of denial.