Mount horizontal 6″ metal exhaust pipe to wall economically

I think you are being too concerned about support of your pipe down where the working space is limited. Attach the hood fan itself rigidly. Secure your round vent damper rigidly to the hood fan. Then secure your vent pipe rigidly to the vent damper. (Through away the 3-1/4" back draft damper.) This will easily support four or five feet of 6" 30-guage round pipe with an elbow and insulation. As soon as it is practical start using your plumbers tape at regular intervals. Insulate the pipe well. The entire length must be well insulated. Leave the insulation light and fluffy. Don't strap it down tight. This kills the insulation value. Use another elbow as you pass through the roof. Try to keep it tight to the roof line. Cut some dove tails in the upper elbow on the top side of the roof. screw them down to the top side of the roof. Install your roof vent cap over it. Now fry fish to your hearts content. P.

You probably don't need to add the fourth vent. If the fan is rated for 100cfm then 4 inlets will pull roughly 25cfm, 3 about 33cfm and so on. As long as the ducting for the 3 vents are of sufficient size and length to handle 33 cfm it should be just fine. I don't have my chart with me at the moment but a 4" pipe pulling 33 cfm should be good up to 25'. A larger outlet pipe will cut back on noise.