"Here's a corollary question: What do we do if (whether or not weapons are found) the coalition's "aftermath teams" fail to foster stable democracies in Afghanistan and Iraq, so that the situations there revert to pre-invasion tyranny, only with new faces?

I think that answer's easy, too: elect different leaders, next chance we get. "

Well, first I agree with all you said. But I don't think it's likely the foreign policy matters as much as domestic policy (politics is always local). And as far as foreign policy, I think the Peace process between Isreal and Palestine is more important than Afghanistan and Iraq. And it's important to us, because it's the most important issue with Arab countries in the region. And I don't know if we will have a successful and stable country in Afghanistan and/or Iraq before the next election cycle. Nor do I have certainty that the peace process will continue forward.
We are a superpower, but our actual influence on these countries is fairly limited, it's largely up to the leadership in those countries.
Though I would predict that it's more than likely that process with be made in Afghanistan, Iraq, and with Peace Plan- all the stars seem to be aligning nicely.
The most important issue for US elect will be the US economy and that looks very promising.
I do think it's likely that Afghanistan and Iraq will significantly effect longer term (+5 years) prosperity in their regions. Whereas Peace process could be done deal within a couple years, and there could be more immediate significant effect prior to or following this peace process.