I would doubt that ice dams of any significance would occur for any roof steeper than 4:12, and plausibly for even lower pitches. Not only is the heated area small, but the heat (BTUs per hour times hours of use) exhausted due to cooking is much less than the household heat passing into an attic in cases where ice dams are a problem. I had a major ice dam this winter at the edge of the roof over my kitchen, but the kitchen ceiling was insulated by only a tarp. However, the question of blockage is an interesting one. A sufficiently wet and heavy (yet blowing) snow might pile up in a way that reduces the aperture of the style roof cap that Broan, say, uses for the larger model fans. However, these fans have overhang at the exit (down side) and I doubt that total blockage would occur. So long as some air can pass, then pushing out warm air should clear a larger hole as needed. I won't be able to attest to that until next winter, but I can say that here in southern NH, when in use a Nutone mushroom style exhaust fan with an air exit point about 10-inches above a 5:12 roof, has never been blocked by snow in 20-some years. If a snowstorm were found to be depositing snow that was suspected of being perverse in the blockage sense, then running the fan during the storm should maintain sufficient air clearance to allow the fan to work when the snow hardens up. kas

As far as I know, no, there are not separate downdrafts that work with anything but a cook top. Are you having cabinets above on that wall? You could use a completely integrated hood and run the duct through the top of the cabinets. It takes a lot more work than building a box but it would look better. You could then run it through the pantry. Look at Broan and Faber Here is a link that might be useful: Broan Power Pack

Is there maybe a closet above that the duct can be run through a chase to the roof? You really need an on site HVAC specialist to explore all of the possibilities here. It may be that running the duct through the top shelf of the cabinets instead of in a soffit is how you have to handle it, but a pro might have other options after a site inspection.

If I understand correctly, the range will be where the current doorway into the hall is now located, which is perpendicular to the window wall (I assume the sink is under the window). You should be able to duct to the left, to the exterior wall. If you are planning stacked cabinets, or cabinets to the ceiling, like the one over the fridge, the top cabinets on the range wall can house the ducting, or you can create a soffit on that wall. Can you post a measured overhead drawing of your proposed layout, with all adjacent spaces labeled? Seeing a layout will help us make suggestions.