"Regarding what happens if you run the battery down to 0%, I don't know.."
What, you didn't do the experiment?
I did it within the first week I had the car, I wanted to find out.
My 2018 i3s rex slows to about 65-70mph on the level when the SOC is very near zero. So it's not really good for the freeway but backroads are fine. Acceleration is reduced and a big climb might be problematic.
This makes complete sense as I have noticed that below about 70 the rex can hold state of charge constant, but if I cruise faster than that the SOC is slowly dropping. So that's the limit of how much power that little engine can make.