Engine revs when letting off the gas

Yes, that is usually the case.
The whole battery never goes bad all at once, but you need a Prius aware scanner or something like Dr. Prius app to find the bad cells.
There are procedures you can find if you search here to replace bad modules and rebalance the battery, but be aware that all the modules are old and you will likely be redoing it at regular intervals.
We call it whack a mole.
How many miles on her? It could be under warranty

 

You need to look at your traction battery SOC. That's normal operations when the traction battery is low.

 

If the dash MPG reading goes to full 100, then the engine is being spun without using any fuel. The car can do this for totally routine reasons that don't have to mean a problem with anything. Can you give more details about the setting where this happens? What speed are you accelerating to, and is the road at that point level, uphill, or downhill?

Can you clarify this point? I thought you had just accelerated up to some speed X and then let off the gas, so I would expect the speedometer to still be showing roughly X. Do you mean it is slowly decreasing back toward zero as you proceed after letting off the gas?

When the traction battery is low on charge, the engine may continue to run, using fuel, in order to add charge to the battery. The MPG gauge will reflect that.

When the traction battery charge is high, the engine may continue to be spun, without using fuel, in order to avoid adding more charge to the battery. From the observation of the MPG gauge going to 100, this sounds more like the situation.

The temperature of the battery can also figure in.

 

Can you please explain how a vehicle accelerates up to speed only for "...speedometer goes to zero." You're literally saying that your car is driving down the road and not going anywhere? And granted by that description you'd get unlimited miles per gallon, but that's not reality.

What's more, when you take your foot off the gas the engine stops revving it doesn't keep revving unless your floor mat is holding the accelerator down... Because you didn't say you're experiencing unintended acceleration, just engine revving when you take the foot of the gas, you're describing a symptom that has never been described before.

Lastly, this has nothing to do with hybrid the battery. Get an OBD2 reader and Dr. Prius app if you want to know what going on with that.

 

Sounds more like unintended non acceleration