Are mountain descents of 4-5 minutes [getting 99+mpg] bad for the battery?

Engine braking will only slow your acceleration, not stop it.

 

One question that hasn't been raised in this string is why the OP is not invoking the B Mode when traveling down those long descents. In the old days, it was standard practice to downshift to a lower gear when going down a mountainside. B Mode is designed for that purpose. Or at least that is what the owner's manual states to do.

 

It does have some light engine braking, though much less than when in B mode. If you put it in N *** to disable that engine braking, you'll find the speed increasing faster.

*** Don't do this if there is any chance the engine (ICE) is not already spinning. It defeats the car's ability to protect itself, possibly leading to overspeed damage or destruction of MG1.

 

In the case of the PIP Neutral means friction braking only - no regen, no engine braking. Is that different on other cars?

(I use this mode all the time as I live at the top of a hill, so have to shift to N every morning to avoid firing the ICE when the battery is fully charged)

 

No, this should be true for all Prii.

I believe the safe maximum speed for this, without destructively overspeeding MG1, is higher for the PIP than for the other models.

 

Big mountains in VT makes me remember going down Killington years ago. Done that many times when I lived there long time ago but never in a Prius. Tricky not to overheat the brakes.
Wicked steep mountain and all the roads to it.