It's possible (more likely not a lot but just some). Another possible interpretation is that he hasn't married her yet.
I think this sentence is normal. I'll understand it as "I'll marry her, but I haven't yet".
I think so. Maybe “vợ chính thức” sounds normal in Vietnamese, but I never heard “official wife” in English. Fianceé doesn't quite fit too.
UPDATE: I just thought it was not a good sentence for exercise before, but now I think I am wrong. ;-P
I think in this context, legal/hợp pháp and official/chính thức bear the same meaning, as to be the official spouse one has to be legally married. but when it comes to translation, I agree they are completely distinct words and not interchangeable.