In the summer you want to have bigger (wider) contact patch, more tire contact with the road for better handling and traction. In the winter it's the opposite - wider tire will not dig in into snow and ice, but go over it , driving on snow/ice means less traction. And I described a situation...
that's correct, all 4 wheels are the same width fit just fine without any spacers. If you want to see how it looks, just remove front wheel of your car and install it in the rear - you'll see that it fits.
As far as I know there are only 2 winter tires available, Bridgestone Blizzak LM-500 and Nokian Hakkapelita and the are only come with one size - 155/70R19.
The "narrow" wheel fits just fine, I drove the set in the winter in my 2015 REX and they were great.
I sold my i3 and have the winter wheels/tires set. Silver Rial wheels with TPMS sensors and with Nokian tires, they are practically new, drove like 300 miles, because my commute is 2 miles round trip and I worked from home a lot. PM me for more info.