How do you know to use the imperfect in the very first sentence? I hear "depuis" and think it's in the present.

The whole story is in the past, so the sentence is saying "had been visiting us" instead of "has been visiting us", which is where you would use the present in French.

But also there's an audible difference between the pronunciations of "rend" and "rendait", and you have to exercise your listening ability...