rationality in my mind is helpful only to the extent (i) an answer exists and (ii) it can be derived.

Or at least approximated. Yes.

If the answer to (i) and (ii) are yes, rationality sounds great.

Lovely.

If the answer to (i) is no, or the answer to (i) is yes but (ii) is no, rationality (or any other system) has no purpose other than to give us a false belief that we're going about things in the best way

I would say, rather, that it has no purpose at all in the context of that question. Having a false belief is not a useful purpose.

This is why I'm asking why you are confident the answer to (i) is yes for all issues.

And, as I've said before, I agree that there exist questions without answers, and questions whose answers are necessarily beyond the scope of human knowledge, and I agree that rationality doesn't provide much value in engaging with those questions... though it's no worse than any approach I know of, either.

You're describing a world that provides a level of certainty such that the rationality model works in all cases

As above, I submit that in all cases the approach I describe either works better than (if there are answers, which there often are) or as well (if not) as any other approach I know of.
And, as I've said before, if you have a better approach to propose, propose it!

I'm asking why you know that amount of certainty exists in the world

I don't know that. But I have to make decisions anyway, so I make them using the best approach I know.
If you think I should do something different, tell me what you think I should do.

OTOH, if all you're saying is that my approach might be wrong, then I agree with you completely, but so what? My choice is still between using the best approach I know of, or using some other approach, and given that choice I should still use the best approach I know of. And so should you.

for the record, my personal belief is that [..] we cannot have 100% certainty

For the record, that's also the consensus position here.

The interesting question is, given that we don't have 100% certainty, what do I do now?