[yldedly]

You've called J and T into question, so let's do B as well. Physicists know that QM and relativity can't be true, so it's fair to say that they don't believe in these theories, in a naive sense at least. In general anyone who takes Box' maxim that all models are wrong (but some are useful) to heart, doesn't fully believe in any straightforward sense. But clearly we'd say physicists do have knowledge.

[Maxatar]

Sure we'd say physicists have knowledge of quantum mechanics and general relativity. And we can also say physicists have knowledge of how to make predictions using quantum mechanics and general relativity. In this sense, general relativity is no more wrong than a hammer is wrong. Relativity is simply a tool that a person can use to make predictions. Strictly speaking then relativity is not itself right or wrong, rather it's the person who uses relativity to predict things who can be right or wrong. If a person uses general relativity incorrectly, which can be done by applying it to an area where it's not able to make predictions such as in the quantum domain, then it's the person who uses relativity as a tool who is wrong, not relativity itself.

As a matter of linguistic convenience, it's easier to say that relativity (or theory X) is right means that people who use relativity to make predictions make correct predictions as opposed to relativity itself being correct or incorrect.

[yldedly]

My point is that QM and GR make very different claims about what exists. Perhaps it's possible to unify the descriptions. But more likely there will be a new theory with a completely different description of reality.

On small scales, GR and Newtonian mechanics make almost the same predictions, but make completely different claims about what exists in reality. In my view, if the theories made equally good predictions, but still differed so fundamentally about what exists, then that matters, and implies that at least one of the theories is wrong. This is more a realist, than an instrumentalist position, which perhaps is what you subscribe to, but tbh instrumentalism always seemed indefensible to me.