1) Clipless? No. I want to use real winter boots and to be able to quickly put foot down if needed. Clipless is also an effective heat sink when the weather gets cold enough.
2) Skiing gloves. Good boots (no sneakers), with e.g. Gore layer. Instead of wearing 6 socks in each foot, get a set of warm insoles. Combine that with the boots, and you will seldom have to wear more than 1 pair of socks.
3) Pair of earmuffs when it's cold. Thin silk balaclava when it's COLD. Both when it's COLD. And a normal summer helmet on top no matter what.
4) A pair of studded Nokian W106. Regarding your question about studded tyres on plowed streets... no biggie. Quality studded tyres wear minimally even on bare pavement (you will notice the rolling resistance, though). Non-quality studded tyres wear fast. Really fast.
5) Salt is not a big problem where I live, but I would be concerned on its effect on drivetrain.
6) No goggles.
7) I use Drymaxx (gore imitation) rain gear with enough layers beneath it. Stay away from anything cotton when considering layers.

--J