Questions about snow tires

Regardless of snow amounts, an all-season tire's rubber is a longer wearing formulation that get's less compliant as temperatures drop. I think the sweet spot is 7 degrees Centrigrade (45 Fahrenheit). If daytime highs are in that range, or heading that direction in the near future, it's a good time to swap.

When we got our Prius within a week (purchase in early November) I got Winter tires on steel rims, and matching plain, open-ended nuts, from a Toyota dealership (just to be on the safe side).

They guided me towards not installing TPMS on the Winter tire combo, I was halfway there already. My 2 cents, they're a money pit. If you do get TPMS on your snows, besides the price, you will need to get the dealership to initialize them (more $'s), and then repeat each time you swap between the Winter and stock wheels (perennial $'s).

If you don't have tpms on the Winter tires you basically have a small orange light on the dash for a few months. And when you put your stock rim/tire combo back on, they'll be fine.

I don't know what's "best", but again just my opinion: I'd steer clear of studded Winter tires.

A 4x4 is still going to get you places the Prius will get stuck, regardless of tires, once the snow gets deep.