I was hoping someone who has "...traversed the Col in winter or with snowpack?" would help you out. All you get is me, I guess. We've been through it many times, but mostly in patchy spring snow. The most difficult part is just before the bottom of the South Needle's north side. You are descending a steep slab of rock with little in the way of foot/hand holds. Depending heavily on veggie holds. That part frees itself of snow early. My understanding is that there is a fixed rope there now. Apparently the route is heavily flagged these days too. We've never found the rest of the route to the Middle Needle that difficult, just steep. Hope this helps somewhat.