Advice on Blown Engine @ 58,000 miles 2014 Prius

I'm hoping to get additional insight into the cause of the blown engine in my 2014 Prius. I think my local Toyota dealer might be trying to brush me off.

Here's the backstory. Back in May, my 2014 Toyota Prius conked out on a trip from Anchorage to Soldotna. No overheating warning lights came on, but halfway through the trip, the engine started to whine. Not seeing any warning lights, I pressed on, hoping to make it to my destination, but the engine failed about 20 miles from town. I had it towed to X Toyota, where it sat for 2.5 months, with the shop saying they had no one to do the breakdown to the point of failure that Toyota required for my powertrain warranty claim. Finally, in late-August (!!!!), X Toyota told me that the failure was caused by a heat exchanger so my extended powertrain warranty is void and my car, worth approx $14K, is a total loss since repairs are estimated at $17K.

Here's why I'm skeptical of the local dealer's diagnosis.

I shared their report with a mechanic I know in Oregon. I've included his notes in bold, with responses from my dealer in italics.
That is all very cryptic. [He is referring to the Prius report, attached.] I would have little confidence that the heat exchanger failed first and caused the engine meltdown. I am not an expert on Prius engines, but this does not sound convincing. If heat exchangers fail spontaneously at low mileage and cause engine failure without triggering a warning, one would expect them to be on the routine maintenance schedule.

Dealer: Vehicle has leaking coolant from tail pipe due to heat exchanger integrated into exhaust that has failed and leaking coolant into pipe. Due to coolant loss from exhaust heat exchanger, engine overheated and head gasket failed and oil became contaminated with coolant. Cylinder walls and connecting rod and crank bearings and jounrnals have been washed out by coolant exposure. Cylinder head and block are warped due to both being aluminum and excessive heat exposure. Recommendation is to replace engine, cyl head and exhaust heat exchanger (catalyst).

Well, it sounds like the technician knows what he is talking about. However, if his analysis of the failure sequence is correct, you would have got an overheat warning light prior to the engine making unusual noise. The fact that you did not raises questions about what came first. I think you would get the same symptoms (coolant in the tail pipe) if a head gasket blew first raising pressure in the coolant system and causing the leak at the exchanger. I still do not think heat exchangers fail spontaneously at low mileage.

Dealer: Sorry for the late response I was off last Friday , and yes you can have mechanic look at just let me know a time/day they'll be here . It is parked outside at the moment but they will still be able to do the inspection . As far as the overheat warning the temperature sensor only works if there's fluid going over it , your vehicle had an internal leak , at some point even briefly the overheat light had to come on and then shut off .

I do not buy it. A moderately slow leak, such as a failure in the heat exchanger would give plenty of time for an overheat condition to register. If not, then the entire overheat warning system is incredibly poorly designed, which I do not believe. The situation the local mechanic describes--only a brief warning light that you might not notice--could, however, occur if a blown head gasket pressurized the coolant system with combustion pressure such that the coolant blew a gasket in the exchanger (and possibly elsewhere also) and was expelled very rapidly.

Any advice on whether I am justified in trying to get a second opinion on the vehicle, or should I assume my dealer is accurate in their diagnosis? I can't tell if there are specific codes that would flag the heat exchanger in the exhaust system, or if they are just making a judgment call. I think it's the latter.

Thanks again for any input.