@Skye_Cleary No problem - it's an interesting question. He also says: "even when Dasein explicitly addresses itself as 'I here', this locative personal designation must be understood in terms of Dasein's existential spatiality" (B&T, p. 119). Haven't looked into it in any detail, but I think he might be basically saying that the factual 'here' is grounded in the existential 'there', i.e. the latter is more fundamental for Being and Time's existential analytic.

@Skye_Cleary It might be because Dasein can mean 'existence' in German, so it was preferred over Hiersein due to the connection with 'ek-sistence', i.e. being out (ek) there in the world. In general, Heidegger likes to create lots of connections between German and Greek terms, so maybe that was a reason.