If I analyse evidential devices, is it lexical or propositional level of analysis? I would say it is propositional level, because, for instance, modal verbs being taken out of context may have different meanings, objective like 'must' conveying the meaning of 'obligation', and subjective like 'must' conveying the idea of a person's logical inference about something. Am I mistaken? Thank you in advance!
What you are talking about are modes of existence, which classify propositions (or more precisely, clauses). From a semantic perspective, eventualities (possible situations) have different modes of existence, some of them being evidential. It has nothing to do with presupposition and only marginally touches entailment (in the case of positive modalities).