P1121, P1123 and P0A983, and mechanic obfuscation

My 08 Prius has 206,000 miles on it. Bought it in 2011 with 35,000 and it has been a great car, never giving me any trouble. Even now, it's just a minor issue but my question has to do with a mechanic. I've had P1121 for years and kept putting off having it fixed. On Sunday, about 9 miles from home, the red triangle of death came on, and a few other lights too. I was able to get home without any trouble and took the car to a mechanic on Tuesday. Been using this shop for six years but this was the first "real repair" they were having to do.

I told him about P1121. He called back in a couple of hours and said "P1121 has been joined by P1123" and said the repair, together with a tire sensor that had been diagnosed earlier would be 1,200 to 1,300. I was shocked because I know the coolant flow control valve is not a big deal repair, and the part should not be more than 200. But because I didn't know what P1123 was, I agreed to the repair. He did not mention any other codes and said the part was expensive and would cost more than half of the total. I should have questioned it then and there or said to hold off on the repair because I know the coolant flow control valve doesn't cost 600. I think he mentioned the inverter pump, which I thought strange so I asked about the control valve and he said something vague like, "I think it's part of that part." Made no sense to me.

I had stuff to do so went about my business but as soon as I had a little time I googled P1123 and saw that it was just another issue related to P1121, nothing that would warrant a 600 part. So I decided to call and decline the repair. When I called I spoke to the mechanic who was working on the car who said it was "all in pieces" and that the repair was already being done. What could I do? I let them proceed.

Less than an hour later, the original guy I spoke to, called back and said that he had been able to get the part for less and the total bill was now going to be around 729-800. He again mentioned an inverter pump. When I asked about the coolant flow control valve, again, he gave again a vague answer. Something about the name of the part not scanning or the part having a different name. Again, makes no sense. P1121 is coolant flow control valve, a known part, no mystery.

The next morning when I went to pick up the car, the repair order listed under Parts "drive motor inverter cooler pump, extended life antifreeze coolant and tire pressure sensor. I asked to speak to the mechanic who told me that he had in fact replaced the coolant flow control valve. When I asked why the repair order did not include that and listed instead the inverter pump, he gave me a lot of vague non-answers, like "it scanned that way" or that's just the name. WRONG!

So I paid and left and at night the CEL came on again. Yesterday it was still on, so went to Autozone to read the codes. It showed P1121 and now also P0A93, which is the inverter cooling system ... At night, the CEL went off.

If you have read this all the way to the end, I thank you. What would you do? (Keeping in mind that there aren't really any Prius shops near me.)
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I'm not sure they do. For six years, they've handled basic stuff, oil changes, tires, fluids sometimes this was the first actual repair and I don't think they knew what they were doing which is why I am reluctant to go back. Even though what you are saying is practical and logical I'm afraid if I go back they will just mess up again and not be able to explain what happened.