I'd take infiltration in order to vent cooking exhaust outside over recirculating in a heartbeat. That said, our vent stack doesn't have a damper in it besides some dinky plastic flaps in the VAH blower box. I might do that differently if I was starting over. Instead, I'm planning to cut a couple of pieces of stainless sheet to replace the plastic flaps sometime soon. We took to stuffing a grungy towel in the blowers during this winter's cold snaps to prevent the heat from going up the stack.

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

If your hood is on an outside wall, your not going to want to protrude through the roof too close to an overhang, especially if you are in cold country with ice dam concerns.You would have to elbow up to a higher location.In our area,a fire stop is required where the ducting enters the cieling/attic with any framing required to be 2" away from the duct.A fire stop is a sheet metal jack that accepts the ducting in the center.The fire stop is attached in your attic to cieling joists or blocking or both.The fire stop isn't so much for heat,as hoods aren't high heat,but are required to slow fire through penetrations.Your roof sheathing should be cut 2" larger than the ducting as well. The roof jack will easily cover that. A t-top roof jack is simply a jack that has a horizontal "hood" that keeps water out of the jack/ducting. You are going to want to study how other vents on your roof were flashed as well concerning your roofing, as an improper flashed jack is a guaranteed leak.If you use foil tape on the seams of your ducting,(my choice), make sure it is rated for heat.Good luck on your project! You might want to check in with your local yocals for venting requirements in your area. That's what they are there for and is a safer bet.

Every bend in the exhaust duct will degrade performance - this is called static pressure. Ideally you want a perfectly straight shot from your hood to the outside but that is rarely possible so you want to minimize static pressure as much as possible. More: https://www.gardenweb.com/discussions/5161173/hood-faq If this is new construction then your builder should have planned for the location of the exhaust duct and not had structural elements in the way. If they did not plan properly or if this is a remodel then you need to work with them to achieve as good duct run as possible. If you cook a lot then I would make this a priority as poor ducting can easily reduce a 600 CFM hood to less than 100 CFM and that's not good.