Learn English – How to use the “that being said”

Given that your source says that it was published in The New Yorker, I imagine the author is American.

In Europe, floors are described as follows: ground floor (street level), first floor (first floor above street level), second floor, third floor, etc.

However, in America, they call the street-level floor the first floor. So if the publication is American, then "first floor" will be the ground floor.

As for "down the hall" it means "along the hall" in this context. So it's further along the hall, not necessarily at the end. It could be at the end, but all it says is that it's closer to the end than the current position is.

Here's a diagram for example:

Doors 1-3 could be described as "down the hall" from a person standing at X. Door 5 is right next to him so it wouldn't be described as "down the hall". Door 4 probably wouldn't be described as down the hall since it's a bit too close, but it could be.

As others have said, it's not a standard phrase. However, it's important to point out how the word heavy can be used to describe things other than weight and mass. Couples can engage in heavy conversations; students can trudge through heavy reading passages. We can be advised not to eat a heavy meal before a long swim.

Your passage doesn't even tell us what is being described – something you should be more careful about in future questions. However, I can venture guess as to what the expression might mean, based on Definition 6 in Macmillan, which says:

heavy (adj.) used about things that look ugly because they are big
heavy features (=large mouth, eyes, and nose) : He was a tall dark man, with heavy features.