I get the impression that many (even most) of the commenters here think that acknowledged contingency thereby undermines a belief. But if you agree with me that this is much too quick, then we face the interesting problem of specifying exactly when acknowledged contingency undermines justification.

I don't know what you mean by "important". I would agree that the instruction manual question is obviously of greater practical importance, e.g. for those whose interest in the theory of rationality is merely instrumental. But to come up with an account of epistemic justification seems of equal or greater theoretical importance, to philosophers and others who have an intrinsic interest in the topic.

It's also worth noting that the theoretical task could help inform the practical one. For example, the post on 'skepticism and default trust' (linked in my original comment) argues that some self-acknowledged 'epistemic luck' is necessary to avoid radical skepticism. This suggests a practical conclusion: if you hope to acquire any knowledge at all, your instruction manual will need to avoid being too averse to this outcome.