I have several friends who live in these types of apartments and all of them have apartments where the elevator opens directly into the unit. This is a common setup.

I've never been in such an apartment, but that is how Robin Colcord's private elevator worked on Cheers, so I believe you.

Do residents never get in the elevator with other tenants of the building? It would feel odd to open my door to complete strangers like that just because the elevator stopped on my floor before theirs.

In all the instances I have personal experience with the buildings are very small. No more than a dozen units and so there isn't usually a big risk of running into anyone. Also, I don't think you can actually ring multiple floors in those elevators. The elevator would need to take you to your unit then go back to the lobby to get the next person.

The building this attack happened in had a total of 7 units.

I'd get a lobby built around that elevator with an additional locked door.. or maybe I'm too paranoid.

This is actually how they are frequently built. Not always, obviously, but it is extremely common for full floor apartments to still have a “lobby” for the unit around the elevator.

I'd say you're spot on. Compromising an elevator can't be the hardest thing to do computing wise. It's all actuators in the end and they really don't care what gives the signal.

Not necessarily. I have not been in the building in question, but have looked at multiple other apartments in NYC where the elevator opened directly into the unit, no door. Typically older buildings, not newer construction.