Would it also be acceptable to say "Arkitekten är i kontoret."?
No, you’d use på.
Swedish use på for location at a lot of venues. På restaurang/kafé/bio/kontoret also many vehicles på tåget/bussen/flyget. You can’t really explain why, prepositions can be quite random.
Then swedish use 'i' for 'a situation', 'a weather'?
So would it be correct to say "på sitt kontor" for "at his office"?
Growing up with an architect father, and knowing how often architects are visiting offices other than their own, it seems like it'd be good to know how to be specific.