especially any fundamental similarities or relations between epistemic and moral justification

So, the fact that in natural languages it's easy to be ambiguous between epistemic and moral modality (e.g. should in English can mean either ‘had better’ or ‘is most likely to’) may be a Feature Not A Bug? (Well, I think that that is due to a quirk of human psychology¹, but if humans have that quirk, it must have been adaptive (or a by-product of something adaptive), in the EEA at least.)

1. How common is this among the world's languages? The more common it is, the more likely my hypothesis, I'd guess.