What is "normal" HV battery SOC?

That is pretty normal for a well balanced battery.

You maybe have misunderstood. The battery ECU will only use the available range of 40% SoC (lower limit) to 80% SoC (upper limit). But it tries to keep the battery at around 60% SoC to allow room to store energy from regeneration but still have enough reserve for demand from the battery.

So, from what you have said, your battery is acting pretty normally.

 

That looks perfect!

Nooooo!!! Unless you want to wear out the battery faster and get worse mileage to boot. The computer knows what to do. Let it do its job.

Actually, the car insists on keeping it in that range. That's how they can offer such a long warranty on the battery. It will not let it go above or below that. (Except that I've read of people running out of gas in Gen 2s and continuing to drive on battery till the battery is totally dead.)

Quite different. The car uses 6 bars as sort of a home base. If you're stopped in a traffic jam and the A/C runs it down to about 2 bars, the ICE will start and charge it back up. Having bars 7 & 8 available gives room for storage of energy regenerated braking or descending hills. I like to think of it a little like a spring that can compress or stretch. If it was kept fully compressed, it couldn't absorb energy in one direction. If it was stretched out, it couldn't absorb it in the other direction.

Here's a very helpful document from @john1701a. John's Stuff - Toyota Prius User-Guide (Iconic)

 

Correct. The indicated range on the display only shows the actual charge from 40-80%.