Why is "odjíždět" used here and not "odjet"? As far as I understand the English sentence it describes a one-time situation and nothing habitual.
Perfective vs imperfective.
Is it possible to use "až" in place of "když"?
No, at least not here. It is hard to find good use for "až" in the past. Those I can think of are translations of "until", not "when".