Interrestingly I did think of norms when you drew up the problem, but in a different way, related to enforcement. We hold each other responsible for our assertions, and this means we need an idea of when a sentence is properly said. Now such norms can’t require speakers to be faithful to all the propabilistic associations of a sentence. That would leave us with too few sentences to describe all situations, and if the norms are to be reponsive to changing expectations, it could never reach equilibrium. So we have to pick some subset of the associations to enforce, and that would then be the “literal meaning”. We can see why it would be useful for this to incorporate some compositionality: assertions are much more useful when you can combine multiple, possibly from different sources, into one chain of reasoning.
Good points! I’ll have to think on this.
I’ve thought about applying normativity to language learning some more, its written up here.