Ok I would keep all the questions in one thread as it helps to see the history.

Re the twin screws I use two motors with out issue. I think it is about half and half ie some use two and others one motor. There are pro's and con's for both.

Do you have homing switches ? generally on the Z axis it only needs a switch at the top. I would set that up as a combined home and limit switch .

You can wire all the limits in series to one pin. ( set that up in ports and pins in Mach) and also set up soft limits as the machine when homed should never hit a limit.

There are plenty of vids on the net:- https://www.youtube.com/watch?v=ma7lMocQbv0 if that helps