I desperately need help figuring out what's going on.

I don't see any videos. you might need 5 posts, idk. I think you can link to a YouTube maybe

 

Can you describe the sound? Have you been losing coolant? If your misfire codes were from cylinder 3 & 4 you'd 100% be able to fix it with coils and while you're at it plugs. However your error codes are cylider 1 & 2 and thats where 2010-2015 have head gasket failures. And go ahead and try plugs and coils, but brace yourself for potential bad news. On the bright side folks with little to no skills have successfully done the headgasket job thanks to youtube videos and help from people on here. Also you can get away with driving a Prius with a bad headgasket for a suprisingly long time before the fail... The risk of doing that for too long is destroying the drivetrain dampener or worst of all a bent piston rod if you get so much coolant in the cylinder that it hydro locks.

 

Did the car have enough coolant in it in the first place?

Did you do your test with the cooling system pressurized?

If not, you may not have valid test results.

 

This is why you picked it up for cheap at the auction come on man you have the standard issue that goes on with this engine you're gonna fix or not . Car is probably Not worthy . Eye of beholder n all that

 

The "change owner" light came on when the previous owner experienced the "Prius Death Rattle."

YOU'RE the lucky new owner.
The GOOD NEWS is that there are a lot of ex-pat Priuses in Aridzona, so there are probably shops available to you that have done the replacement.
Verify the HG good or bad with a leak-down test using a borrowed tester from an auto parts store.
Since you have a bi-directional tester you can also test to see of the EGR is working.

Good Luck!

Read Fast!!!!
PC is swirling in the drain as I type this........

 

It's quite hard to see latent leakage, which is why it is important to pressurize the cooling system with a test kit while you are doing the video inspection.

A secondary data point is the appearance of the piston crowns: dirty ones with a good bit of carbon generally indicate normal conditions when the odometer is that high. When you see something clean and shiny, that can indicate that steam cleaning has been accidentally occurring, using steam from your coolant.

In any case, you may be fine- a Prius with 200k miles on it would normally be almost due for its 3rd set of plugs, so hopefully that's all it was.

Beware of cheap counterfeit plugs and coils found online. A few other priuschatters have found out the hard way. Worst case so far was a plug dropping part of the electrode into the combustion chamber.