Answer

You are not distinguishing here between idolatry and heresy. God and idolatry are two different things. Heresy is incorrect belief; idolatry involves problematic religious worship. The prohibition is against idolatry, not against heresy. (True, there is Maimonides’ first positive commandment, but it deals with belief in one God and not the details, and beyond that I have written more than once that one cannot command beliefs and facts.)
From your question it seems that you are dealing here with mistaken belief and not with idolatry. The question is: what practical difference does it make whether the belief is mistaken or not? For example, there is a dispute whether the Noahides are commanded regarding associationism. If they are not commanded, that does not mean that associationism is correct, only that they are not required to abandon such a belief. In other words, there is no punishment for holding such a belief. But in my opinion there is no punishment for holding beliefs at all (perhaps one can punish for negligent inquiry).
The religious worship of Christians, even if they direct it to one God, can still be considered idolatry, because it is a religion they invented out of their own hearts. What determines this is not only the object of worship but also its form (whether it is grounded in a commandment).
That you should ask them. In my view, the Trinity is an empty slogan, and one cannot determine that someone who holds by the Trinity is a heretic. (Isn’t it like associationism? One can debate that.) The question is what meaning they give to the Trinity. There are interpretations that definitely come close to various interpretations in Kabbalah (the Holy One, blessed be He, Israel, and the Torah are one). But that does not necessarily mean that Christianity is fine. It could also mean that Kabbalah is not fine. Of course, someone who believes in it believes in it, and does not care whether I think it is fine or not.