Can someone explain how this makes sense electrically?
However, if I only turn off the lights breaker, and go into the switch panel with my voltmeter, I am still detecting 100V AC across the switch (with it off), even though the lights stay off when I turn the switch on. … Can anyone tell me what might be going on?


Coupling between wires (inductance and capacitance in parallel wires).

You say you have strange long runs between outlets and switches — if any of these extra lengths run in parallel, this is a great environment for the voltage from one to be induced in the other if it's disconnected.

If you try putting a load, like a small light bulb, across that 100 V, it will vanish (read 0 V), because there is no significant current capacity in this coupling. It might even go away if you were to attach an analog voltmeter (which is more of a load because current from the line to move the needle) instead of a digital one (which typically has a 1 MΩ input impedance that presents almost no load).