Could a P1121 actually be TPS issues?

Um, is there any reason you even think there is a throttle body issue?

 

Negatory on P1121 meaning anything but the coolant valve. If you look it up in the repair manual there is no confusion.

It's in a manufacturer-controlled range of codes, so if Toyota wanted it to mean throttle something something in some other car model they make, they could, and that could explain generic lists or Google getting it wrong, maybe even Techstream (though that would be a bit embarrassing, since Techstream knows what car model it's talking to). But in a Prius, it's the coolant valve.

I'm not sure what you're getting at with TPS having redundant sensors ... yes it does, and I think they are arranged to read complementary values, where the pair of numbers you reported would probably be normal.

 

Yeah, it has tended to be Toyota's practice to build in two potentiometers that traverse the 0–5V range in opposite directions, and the ECU confirms that the readings are mirror images. Sometimes they'll use a different sensor technology than actual potentiometers, like Hall effect sensors, but still keep the dual-opposing-signals strategy. I guess that does make it easier to catch the sort of problem where both signals get shorted to +5 or to ground.

The Gen 1 Prius did use actual potentiometers in several places (accelerator position, steering torque, and HVAC flap positions), and any old enough Gen 1 has probably dealt with at least one of the related nuisances when the pots became electrically noisy ("Big Hand Syndrome" when it was the go pedal, "steering gear jitters", or "no, that wasn't a mouse in the heater" from the sound of the flap servos jittering around). By Gen 3, all of those three had been replaced with non-potentiometer sensing technologies. I think Gen 2 still had real pots in the HVAC servos.

Congrats!