but for real-life cases, how could accurate estimates of P(A|X) be obtained?

In order of (decreasing) reliability: through science, through expert consensus, through crowd-sourcing, through personal estimates.

but what exactly IS a world-model?

Simply the set of sentences or events declared true. For a world-model to be useful those sentences are better to be relevant, that is, can be used to derive probabilities of the questions at hand.