as... relates to a previously mentioned person/thing. The case of B depends on the case of the mentioned person/thing it relates to.
He [nominative] acts as a B [nominative].
He presents his [genitive] own position as B's [genitive]. (as... relates to 'he' in 'his' not to 'position'.)
He sees her [accusative] as a B [accusative].
He pays tribute to her [...