Long time lurker here. I'm sorry you're going through a hard time. It seems like the factual information you really need is about how chemicals would possibly leave your milk, and their duration. Other than that, it seems that you will just have to wait until this gets resolved, however you decide to do it. If your milk supply is tanking, it just "is what it is" until you can maybe change that.
I'd also like to suggest the possibility that your milk may not have taken as much of a nose dive as you worry. You know best, but do you think it could be a growth spurt? That's what frantic round-the-clock nursing has been in my experience. Or teething or illness.
I'm also so sorry for your breastfeeding struggles. Congratulations on working so hard and overcoming so much. I guess my immediate thought at that struggle is that it is now a sunk cost, that is, it is something that happened in the past. And while it is emotional for you, if you are making a factual decision it does not really change any decisions you are faced with making now. You just have to do the best you can in the moment. I understand how it would feel like an attack on your current situation. I'm just trying to point out another view.
It's my understanding that you can comfort nurse no matter what the supply is like. You seem to be saying that your goal is to breastfeed at this moment. I hope you are comforted by the fact that you are nursing, and your babies are getting so much good and closeness at this very moment no matter what. Maybe if you could clarify your goals further it would help you. Breastfeeding for one more month, 6 months, 2 years, until it just doesn't seem like the thing to do...? I hope this was constructive and supportive. Best of luck to you, and peace. I'm sure everything will work out, even if not ideally, then fairly well.