~125vac from every terminal, to ground?

But still 0 volts between the top two terminals?

That's consistent with one of the two live wires being disconnected somewhere between the heater and the breaker supplying it.

I said earlier that for this scenario to be correct you'd expect 0 volts on one side while only having ~120 on the other, but I forgot you'd read voltage through the heating elements because the thermostats will be calling for heat and connecting everything in the heater together.

The best way to confirm this would be to [with the breaker off] disconnect either the black or red wire from one of the top two terminals. Pull it out a bit so it won't come in contact with any metal, but you can safely probe it and the terminal it was removed from. Now turn the power back on and check voltage to ground on that wire and it's terminal.

The black and red wires on those top two terminals you've been checking are the power comming in. Regardless of what the heater is doing, even if it's totally broken with elements rotted through, you should be seeing ~240vac between those two. And both individually should have ~120vac to ground.

Everything I'm seeing so far tells me electric supply problem. Perhaps a switch like I mentioned, they can be in not very obvious places. Could also be a failed breaker; it should be a double pole breaker that takes up two slots in the pannel. Sometimes one side will feel squishy, like it's not closing/connecting properly.

/edit:

From these tests I believe I can conclude that all of the components (both thermostats and heating elements) are at least receiving power from the grid.

While each component is receiving power, they are only receiving half of a 240v supply. Note you cannot find a voltage across any component, only voltage from ground to each component.