Q re passe compose

Aussitôt introduces two actions, of which the first one has already completed before the second one starts. Since both actions happened in the past, the first one requires the plus-que-parfait (étions arrivés).

Andy, 

As Chris notes, the tense used is plus-que-parfait parfait, which conveys the order of the events being discussed here. 

I think it is better to start from the likelihood that the tense used in French by the native French writer, and speaker, is ‘ correct ‘ - only rarely will it be otherwise, especially on this site with the staff having professional qualifications. From that assumption, when uncertain, I try to understand what the tense they have chosen tells me, compared to any alternative I think may be possible. 

There are situations in which there is a choice of tense depending on the message to be conveyed. Although it discusses imparfait and passé composé, the attached link is one of the most practical discussions I have come across on the use of these 2 tenses. It helped me develop a ‘feel’ for them, and understand that the choice is not always set in concrete !

https://www.frenchtoday.com/blog/french-verb-conjugation/passe-compose-versus-imparfait/