Sounds like a really difficult situation.

Firstly, I understand your desire to help your cousin but you shouldn't put your relationship with your fiancee on the line because of his actions. No one should be expected to undergo that level of self-sacrifice so whatever happens, don't feel guilty about your decision if you decide to stay. You can still help your cousin without him staying with you.

Secondly, addiction is just a horrible condition. But, your cousin was successful in quitting once and he can do that again- relapse is a part of addiction and kicking the habit on the first try is rare. A month clean from duster is a very reassuring sign that there is a part of him that does want to quit and stay clean. From your post it sounds like a lot of the discussions about his duster habit happened in the context of belligerent confrontation in one way or another: that might make it difficult to get the message home since in those situations people tend to double down and entrench themselves in their own views. The relapse seems to have happened because of his contact with his wife, the fact there was a trigger for the relapse is another reassuring sign, since it means that external factors were what caused him to start using duster again and not his own lack of self-control or motivation.

You need to talk to your cousin in a safe setting where you both have the time to discuss what's going on. Tell him everything that you're feeling, explain to him the consequences of using duster like this- both for his relationships and his own health (the shit is seriously bad for you), reassure him that you will be there to help with whatever you can but you won't put your relationship with your fiancee at risk. I bet he feels isolated at the moment, having been thrown out, and that will only make the situation worse. Try and convince him to give rehab another try if he can afford it, make it clear that relapses are normal, common and in many cases can contribute to successful recovery if you learn the lessons, i.e. what triggers you to start using again and how to avoid them.

It sounds like your cousin has a lot of contact still with his old lifestyle of drug using. Being around other drug users like his wife will NOT help him at all, for now he needs to stay away from them- not permanently but at least until he's stabilised off his drug use. Weed alcohol and acid sound fine so good call there, but pills and inhalants are an absolute no go in his case.

Finally, you are not responsible for what happens to your cousin. I get that you feel the need to help him and it's really impressive that you feel so invested in his recovery... but if things don't go well that's not your fault. There's only so much you can do for people suffering from addiction. Based on this post and the way you write about your cousin I am pretty sure that any other relapses on his part will absolutely not be because you didn't do enough to help him.