I looked into installing a single wall oven under an induction cooktop (I ended up getting an induction slide-in range though). What was interesting was that most induction brands allowed it IF the wall oven was the same brand as the cooktop. This included Kitchenaid - don't know where you got your information, but the installation instructions do allow for it with certain ovens - it turns out it is the Kitchen Aid single ovens. Two brands did not allow for a single oven under the cooktop even with their own brands (well, it voided the warranty so the same thing): Bosch and Thermador. No problem with Jennair, Kitchenaid, GE, Electrolux, and Frigidaire. I think Whirlpool also didn't allow it, but I wasn't considering that brand. Don't worry about the venting. There is some venting in all ovens - whether built in wall ovens or ranges. The vents aren't any more hot for a wall oven than a regular range. The reason I chose a slide-in induction range had nothing to do with the height of the oven. The wall oven under the cooktop would have been about the same height as a range's oven. Rather, I really liked the Kitchenaid induction with its (albeit small) second baking drawer. I cook a lot and could see using this. Plus, the cost was much less for a slide-in than a separate cooktop and wall oven. Our house is being remodeled and we haven't even been able to move in yet - so I haven't used it. I hope I love it. It does look nice.

I am in the Midwest also and paid a reputable plumber $300 to run the gas line from the furnace up thru the basement ceiling and over to the back of the stove. I believe it was also him who hooked up the range to the gas & everything. I had started out with 2 smoothtop electric ranges, yes, 2 because we had scratching issues with both, so the 3rd time, I said the heck with it, I'm switching to a gas range that I can at least clean without worrying that I'm wrecking the top of it every time. Also, I had not cooked on a gas stove since early marriage 30 years ago, so I had to relearn it a little, but I can tell you both my DH and I both like to cook and we both really like the gas range. Also, here, we were told that a plumber would have to install a gas line. We also just put in a gas fireplace so the gas line was already there for that so we kind of killed 2 birds with one stone. Hope this helps!

Another choice for the OP is to purchase a 30" rangetop instead of a new 36" cooktop. A rangetop provides more cooking real estate than a cooktop because the knobs are not taking up room on the cooking surface. It won't give her 6 burners, but a rangetop will allow all 4 burners to be used with larger pans at the same time, which is not what a 4-burner cooktop will allow. Not many people actually use 6 burners at once, but the extra elbow room is appreciated. A choice of a rangetop will also be an easier installation, as the cutout is simply the 30" width, and not some specific measurements that are unique to a specific cooktop. And finally, the upper cabinets can remain the same, with no issue of a 36" cooking surface under combustible materials. The only downside is that the OP will have to lose the drawer directly under the cooktop, in order to fit a rangetop. That is still a smaller price to pay compared to reconfiguring the upper cabinets in order to avoid a fire hazard. You can find a less-expensive 30" rangetop at a scratch-and-dent or display sale, or on Craig's List.

I am in the process of making an induction cooktop purchase decision, and was surprised to note the venting requirement when I reviewed the on-line installation manual. I found this posting as part of trying to make sense of the requirement. I like the creative idea from Sativa McGee Designs. However, unlike Sativa McGee Designs, my reading of the installation manual is not that having the vent space is a mere "suggestion." Rather, the directive is proceeded by a capitalized and bolded "IMPORTANT" and is grammatically written in the imperative as follows: "IMPORTANT: For proper ventilation, provide a vent of 3/16” (5mm) under the countertop, in the front of the cabinet. The ventilation opening is to extend the full length of the cooktop cutout.” I would be interested in knowing of any other novel approaches to meeting the requirement. I also plan to contact Kitchenaid for their input.