If it is in principle impossible to explain why one ought to do something, then what is the function of the word "ought"? Straightforwardly, it can have none, and we gain nothing by its existence in our vocabulary.

People are more complicated than you're modeling them as. People have numerous conflicting urges/desires/values/modules. Classicists would say that 'ought' refers to going with the virtuous action; Freudians the superego; Hansonians your far-mode. All of these groups would separately endorse the interactionist (psychology) viewpoint that 'ought' also refers to social pressures to take pro-social actions.

(On a side note: it is completely possible to explain why one ought to do something; it merely requires that a specific morality be taken as a given. In practice, all humans' morality tends to be similar, especially in the same culture; and since our morality is not exactly like a utility function, in so far as it has conflicting, non-instrospectively available and changing parts, moral debate is still possible.)

simultaneously to assert that one ought to believe this

Well, yes, one would need the additional claim that one ought to believe the truth. Among humans, for specific cases, this usually goes without saying.

Everyone accepts that questions relating to the life cycles of stars are questions of fact about the universe (questions of epistemic rationality), but the philosophical pedigree rejects the idea that questions about what is an appropriate way for a person to behave are similar (instrumental rationality) - it seems that people are somehow not part of the universe, according to this wisdom.

No. Would you also argue that you universe is not part of the universe, because some people think it's pretty and others don't?