Head gasket blown. Pink/orange tapioca. Have a few questions as to cleanup before proceeding.

Yes, you made it very confusing.
Now it's too much to read.

Hope that whatever it is doesn't happen again and that you don't have a bad head gasket.
You should change it BEFORE it fails, when you can decide the time and place.

 

Please, stick to the same thread.

 

Well, you thought you got it all out, but it got flushed out.
Soooooo.......
It's just pooling in the reservoir. It's throughout the system.
Until you clean/replace every hose and part in the coolant system, it likely won't go away.

Doing what you're doing, eventually, you'll have flushed it enough, that it will eventually be cleared out.
It all depend on how many time you want to do what you are doing.

I believe Toyota has a coolant flush. Perhaps you should use that, then flush that out and fill with the
Toyota 50/50 coolant???

 

Third time is the charm? Remember the coolant reservoir is an active part of the coolant circulation as soon as the thermostat opens. As a result any contamination is quickly distributed through everything including the heater core regardless of heat or cool settings at the time. When driving it takes about 5 minutes for full circulation to begin.

Toyota coolant is made to prevent internal corrosion and clogging. It has a role in protecting gaskets and seals including the head gasket.

Since a coolant system drain and fill leaves about half the fluid each time, I have seen shops do multiple drain and fills using distilled water with the car driven each time to dilute the contamination (usually sealer) more each time. Maybe ten drain, fills and drive cycles. All done in one or two days.

At that point, one more drain followed by a fill using straight Toyota concentrated coolant. From then on, 50k mile drain and fills each time using standard procedures.

 

If I were a betting man, I would put money on a head gasket issue assuming no one was punking you by directly adding oil into the coolant.

However, your situation is unique in my memory over the last twenty years of Prii interests. Which is the reason you did not get absolute assurances that you had a fluke of nature. Plus we were not in front of it with a borescope and coolant system pressure tool.

We would all love to get assurances that a simple and low cost solution is all we need. Perhaps you also wanted simple clarity when you expressed some mildly snarky (frustrated?) comments along the way. Of course you could always ask for your money back and get a full Priuschat diagnostic refund instantly. The dealer or other pro shop would have diagnosed faster and more accurately but refunds of their time never happen even if they are proven wrong. On a forum you will get conflicting advice; it's the real cost of a free consulting service.

If the contamination was immaculate conception* with no human skulduggery, perhaps the prophesies are getting ready to happen. If not we will wait for another update which would not be uncommon on a gen3 Prius of a certain age.

*loosely defined