To clarify, in my hypothetical I meant to ask what would happen if I demonstrated true moral obligation to your satisfaction. If we could agree that this fact was true, why WOULDN'T the next question from your skeptical posture be "Why privilege truth?"
There is a difference between epistemic norms and moral obligations.
Epistemic norms govern when people ask questions, offer arguments, attempt to persuade others, or, more generally, the entire practice of inquiry. These things implicitly operate under rules like "truth matters", "contradictions are problems", and "evidence is relevant".
Those norms are not being introduced as moral obligations, rather they are preconditions of rational discussion. Anyone who enters into argumentation necessarily relies on them, whether they are aware of or acknowledge them explicitly or not.
Pointing that out does not establish anything like a "metaphysical obligation", it's merely identifying a performative constraint of argumentation.
That is an example of the kind of codependent ontology I'm arguing for. There is something embedded in the nature of "truth" that makes adherence obligatory for us.
Not true. What follows from the argument above is much much weaker than that.
Argumentation presupposes epistemic norms, that's it. If someone rejects those norms, they simply exit the practice of rational argument. What they are doing may still qualify as talking, persuading, bluffing, or asserting things, but it is no longer argument in the sense governed by those epistemic standards, and neither do they incur some sort of metaphysical obligation.
It's entirely possible for people to lie, ignore evidence, or reason badly. But what they cannot do is coherently argue while denying the standards that argument relies on. That is a point about the structure of reasoning, and not about moral obligation.
We seem to agree that there is a such thing as a metaphysical obligation.
No, nothing said so far establishes that.
What has been identified is an epistemic constraint within a particular activity. That is very different from saying that there exists a metaphysical obligation binding on agents regardless of what they are doing. To make use of an analogy, it would be like concluding from the fact that the game of chess has rules that there is some sort of metaphysical obligation related to chess-playing. The fact of the matter is that if the way you play doesn't conform to the rules of chess, then what you are doing is not playing chess in the sense defined by those rules.
If you agree with this characterization, I can try and move from here to moral obligation
By all means, try, but I doubt the step from "feature of a practice (in this case, argumentation)" to "categorical moral obligation" can be explained or justified.
Showing that argumentation presupposes epistemic norms, or that the game of chess presupposes distinct rules for how pieces may move on the board, does not by itself establish that moral obligations exist, let alone explain what property of reality would make violating them a moral failure.