How to quantify the cable impedance for common mode voltages?
When you're using a coaxial cable, you're not modeling the signal as differential.  The shield is assumed (on both sides) to be ground.  There's a lot of definitions of ground, but I'm using the most useful one...zero volts.

In reality, the lightspeed limit means there will be different potentials on the two ends of the coax, but on each side that potential is defined as zero volts.

The difference between a single ended connection and a differential connection is whether you compare your signal potential to a different nonzero potential, or to ground.  It just depends on which model works best for you.  You could, of course, use a coaxial cable to transfer a differential signal, but you'd lose the benefits of having a grounded shield (in this case "grounded" adds the additional implication of low impedance to circuit ground) and your only reward would be having to do a lot of additional math to work out the EMC implications.