Temp Sensor / Serpentine Belt / Water Pump Pulley Help

Wow. Do I know how to have fun! Moving, roof, hvac work, electrical... now my amazing little 2004 Prius (120k miles) that has been a total champ has decided to jump into the chaos. I'm not shy about working on my car at all. I've replaced the inverter pump in the past, changed the serpentine belt, several other random things.. but this is feeling like a bit much at the moment so I'd love some help troubleshooting this.

Friday night just as I hit the highway I got the big red triangle and the overheating light. I got off the highway immediately and as I'm waiting at the exit the dash error lights turn off. Of course I know all is not well, but that was interesting. (no overheating smell, btw) I made my way slowly to a parts store to get a code. Meanwhile, the lights come back on if I'm using the motor uphill or if I hit about 40-45mph. The store's reader shows no code at that time and nothing was lit up. So I make my way to my place and just as I'm arriving I finally got the check engine light at which point I went directly to the local parts store for a code. (P0117) So I was excited it's just a sensor - except that when I opened to hood to really investigate Saturday in the daylight I realized the Serpentine Belt is not there anymore. I found it curled at the bottom of engine compartment and it looks pretty mangled. Now I'm even more excited because replacing that is a snap and it could be causing all of this. But it's strange that it's mangled. I also noticed that the coolant reservoir sitting at the top of the engine looks full. The one by the fan looked so incredibly low I couldn't tell if there was any in there, which didn't make me feel great, but I filled it.

I replaced the belt and filled the coolant reservoir and hoped all would be well only to have the lights come on again about 2 minutes after I hit the highway again. There was a huge downpour yesterday so I investigated the car this morning. The brand new belt is broken and the reservoir looks to be nearly empty. (though I'm parked on a hill with the car facing uphill, in case that info helps. Also, there's no fluid on the driveway at all so if it's leaking I'm not sure where it's going)

Today I see new details to share and I'm baffled, so I've taken photos. The tension pully is working just fine. It moves easily, I can loosen and tighten it. That seems good. But I see now that the water pump pulley is not only NOT turning at all, but it's also rotated outward and not in line with the other ones. I don't believe that was the case yesterday but I can't say for sure. (it explains the mangled belt though!) So, it's not turning and also not in line. The 3rd pulley, the crankshaft damper I believe, is also not turning at all. I assume it should. ?

The car is nearly 20 years old but not crazy high mileage and it's been a fantastic car. I'm not prepared to give up on it but I realize there's a time I will have to of course. But I hope now isn't the time. That said, before I go buy a new water pump/pulley situation, a new crankshaft pulley, another serpentine belt, etc. I'd love to know if any of you have experienced this? And/or if you know what the main (root) issue may be that would be great. If it's just that these several parts need to be replaced or maybe one caused a cascade effect, I'll get busy on the fixes. But if this is likely some other major issue and I'd only be patching it, I'd be most grateful to know that.

Oil is clean and level is good too, btw. So I'm not missing any oil. Just coolant. And to reiterate, it never smelled like it was overheating but I made a lot of stops and stuck to city driving during the weekend fiasco.

Thanks in advance! (I have pics but can't upload or post a link.. sorry!)

 

Dolj & mr_guy_mann, you two rock. I'll be getting on all of this today. Thank you!