Flushed ICE Coolant and only managed to drain 5 quarts?

Normal. Your year has a vacuum break up top in the back but most just refill with the amount drained. You will never get it all out so the smart move is to use Toyota premixed for compatibility.

 

2010 prius here; and using only the Radiator Drain Cock Plug, and "venting" it by opening the screw valve above the egr cooler (as well as the overflow tank cap), I was able to get about 5 quarts as well. 2011+ don't have the valve, but can disconnect the hose up there. I actually don't think the venting really helps much, in any case. Picture shows the white screw cap above the EGR cooler.

 

You might get a bit more draining the exhaust heat recovery component. I did engine coolant drain fall of last year, considered doing that, but all the hoses looked so skookum, I thought let sleeping dogs lie.

Also, cus the Repair Manual told me too, I tried draining the block. The spigot for that is WAAAY round the back, really hard to get to. And about 2~3 tablespoons came out, a total wild-goose chase Toyota...

Just checked my notes, I got even less:

- engine coolant drain: 4.25 liters roughly
- inverter coolant drain: 1.5 liter roughly

 

I changed mine 2 years ago. Don't remember how much I put in, but it was way less than the stated capacity.

I also changed the radiator and inverter coolant of my 2012 Prius C. The specified capacity is 7.3 (5.6 + 1.7), but I only drained about 1 gallon.

Somehow the difference between coolant drain and the specified capacity is much bigger for Prius than for other cars I have worked on.