I looked at my picture on my old sending unit. It had screws and I did remember it saying empty and full. However, I pulled my panel to get access to the tank to adjust the new unit ( I was going to calibrate it with a tall glass of gasoline. ) My new unit does not have adjusting screws. So far this unit seems to be much more correct than the original. I have put 15 gallons in and it is not even 3/4 tank yet. My original would have said just under full.
As far as the aditional gauge. These are my thoughts. It would probably be easy to put in a mechanical gauge. But it would be a lot of work if you want to keep both. One, there is not an extra slot in the tank. You would probably have to cut a hole and your are not going to want the debris in the tank. You would also have to run new wires.
If you could find a place to mount a mechanical gauge, (I doubt a 2013 has a 2-1/2" hole available as they are going to screens these days) you could easily use the slot and the wiring if you are willing to give up the other gauge.