Honeyhas6

As much as I like down drafts, for your situation I still don’t have a good idea to make use of both fans. The range exhaust at 500-600 cfm on max is typical design for these things which is a great option to have to remove heat. For all practical purposes All pressure ductwork will leak a little, all of it, always, all the time. Do your best to seal it nicely, maybe foil tape all vertical seams, and joints including elbow joints to minimize to well below 1%. If this duct extends above the garage into an unheated attic space, you may need to insulate it to prevent winter condensation from forming and rusting out the ductwork. Looking at your southeast location, it may not be an issue.
Save the downdraft for another day and improved design using a proper mixing manifold mounted on the kiln and next time enough dilution air to keep it running cool with that particular axial fan which uses its own exhaust to keep the motor windings cool.. If he watches the video above, that fan easily can serve two kilns and a full exhaust grill to ensure enough air is present for cooling the motor and ductwork. If that fan only draws air through a couple small holes it will be starved for cooling.
The simplest application I have seen for this type of (axial) fan is a full open tee below the fan with the duct running down to the kiln. Even with such a large hole in the duct it will still draw several tenths of an inch suction at the kiln mixing plenum.  You only need a tenth of an inch (-0.1” wc (suction)) at that point to perform like the store bought.

My understanding of the physics (fluid dynamics) behind the downdraft vent is that the box under the kiln (L&L calls it the bypass collection box, Skutt calls it the plenum cup) has an opening on the opposite side from the hose connection so that the fan at the output on the far end of the system can pull a significant volume of room air through the box and thus create a venturi effect to draw the kiln fumes down through the holes in the base of the kiln. The L&L collection box has an adjustable "sliding valve" to regulate the amount of room air drawn through the box (thus adjusting the venturi suction across the holes in the kiln base) while the Skutt plenum cup just has a hole. This flow of room air also serves to cool whatever fumes are being drawn out of the kiln and up through the fan. The system does not apply direct suction to the holes in the kiln.
Because air flows seek the path of least resistance, the hole in the pipe of your vent will both increase total air flow at the fan and reduce suction at the base of the kiln when everything is in good condition. However, it introduces the necessary cooler room air at some distance after the fumes leave the kiln. Also, your design with the fan in the middle of the duct run provides the appropriate negative pressure from the kiln to the fan, but applies positive pressure after the fan, blowing the exhaust into the stove hood in hopes that it will be picked up. As Bill notes, this may not work, but also as the duct work corrodes over time (it will, count on it), the positive pressure will blow the stinkies back into the room long before they come out by the stove hood. The design of the Skutt and L&L systems places the fan right on the outside wall of the room so that the whole system remains under negative pressure until being exhausted to the outside. Then, any minor leakage in the duct work will draw air in, not push the fumes out.

A downdraft vent is good because it will increase element life and decrease corrosion of the kiln's metal parts. However, like Bill said, having it blow out into the hood is probably not very effective at removing the fumes from the room. With big powerful hoods it'll work pretty well, but not so much with a cooking hood. The hood is not going to catch everything. I would run the downdraft duct all the way to the outside. As for whether or not it needs more fresh air holes, you'll need to measure the temperature of the air in the duct. It should be under 140F. It's a balance between pulling enough air from the kiln to remove the fumes, but not so much that you're bringing too much cold air into the kiln and making it work harder. Ideally, the fresh air hole in the duct should be right at the kiln. Otherwise you're pulling really hot air through that flexible duct, which isn't safe and will degrade the duct very quickly. You're trying to keep the ductwork cool as well as the vent motor.