I think that your instincts are very correct as far as not getting in the middle of this - you're not in a position to be any sort of mediator, clearly. I think A should also look for advice from a more neutral person (or people, preferably poly-friendly, of course) because again just the existence of your relationship with A is now opposite of her goals for monogamy, so any ideas and advice that B sees as coming from you is going to seem as suspect, and not given fair consideration.

Which is all very hard, because you do seem to have a honest and admirable desire to help A repair his failing relationship instead of sabotaging it, but there's no getting around it, you can't be seen by both as a neutral, non-biased party by both of them.

On the other hand... I think you're entirely too self-sacrificing in thinking you should "bail" on the relationship between you and A because it's causing problems with B - assuming you still want to continue otherwise. I think first off that it would be very disheartening to A if both his partners vanished under these circumstances, but more importantly it could set a precedent that B can in some ways control or influence A's other relationships, and that's not at all helpful in he's committed to staying polyamorous (as seems to be the case) or even if he decides to try monogamy for some reason - it's much more likely to stick if he feels he's made a conscious decision in that direction, instead of simply having no other partners left.

Again I suspect that will be hard for you, and you might feel as though you are tearing them apart, but remember that's not the case - the problem is B want's A to be monogamous with her, and A doesn't feel he can live monogamously - which will always be an issue even if A never dated anyone else, and emphatically has nothing to do with him dating you in particular.

So my advice would be to be supportive of A, and also supportive of B, as much as possible, but make it clear to both of them that while you have compassion for their problems, that it's something that they have to work out between themselves. You can give them the space and flexibility to do that, but you can't really help beyond that. I do hope things go well and some amicable compromise is reached which only deepens A and B's relationship and I encourage you not to give up on that hope either! Even so, it's entirely possible that they are simply incompatible, in which case remember that it's better for them to work that out sooner rather than later, as dismal as it may seem at the time. Even though we view divorce as inherently bad, it's not uncommon for people to split with at least some dignity and both end up happier as a result. (Actually I knew a guy in college who told me his parent's got along much better after their divorce than before - to the point that people occasionally mistook them for a couple!)