I have the 120 Advantium installed under the counter in an island. Works great have not had any issues. We really did not have any other logical place to put it. It has turned out to be a perfect location. From everything I could tell the only reason GE addresses is for liability. With small children it makes it easily accessible to them. The only possible issue I have encountered is that it is also nose height for my dog. She generally is not in there but once in a while she tries to get sneaky. There is a trade off in that you must bend over or as I have learned to do with my back squat to put things in or take them out. On the flip side it is easier to remove hot dishes and lift than it is to lower them from a taller location. Ideally I think counter level would work best but then most of us don't have ideal conditions.

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.

Miele makes a speed oven that also has a microwave function and it can be mounted under the counter.

I don't think it would pose a problem to the counters, but I don't feel comfortable answering that. If you call Miele, you will get good advice, I think. The ventilation is for the electronics, NOT the steam. I retrofitted a combi steam in an existing cabinet, and I had to research that question. Basically, you need air circulation for electronics behind the oven.