Terminating Resistors on Ethernet Port
The whole network of resistors is the termination, not just the 120 ohm resistor. So the differential pair is terminated with approximately 109 ohms, which is close enough to 100 ohms. When both ends of a differential pair are terminated properly to 109 ohms, indeed the total termination is 54 ohms. If you use some pairs for power, they are not unused any more and don't need termination, as voltage source drives then with almost zero impedance. Completely unused pairs can be terminated, sometimes there a 50 ohm resistor to ground on each unused wire.