When exactly did Avraham realise the 'men' were angels? He does not register surprise when one of them asks for Sarah by name, despite their appearance as strangers.

Also, I find the word 'nitzavim' to describe the 'men' standing before Avraham puzzling. It is used in the sedrah of Nitzavim to denote the Bnei Yisrael standing before G-d. Why not use the word 'amdim' instead? And if they were standing before him, how comes did Avraham have to run to the gate?

Any ideas?