Got a Kenmore OTR microwave to replace a GE OTR microwave. Already removed the GE, but I just noticed that the Kenmore is quite a bit deeper. That's no big deal, but my trouble is that the top cabinet mount bolts for the Kenmore line up with cam nuts that keeps the cabinet together. I can get the middle bolt in though. I have heard most of the weight is supported by the wall mount. Do you think I would be ok to only mount the microwave with the wall plate and only 1 bolt on the top cabinet?
While most appliance mounts are deliberately over-engineered, leaving out two out of three support bolts seems to be a bit risky.
There might be another solution. If you have enough clearance over the stovetop, you might lower the mounting of the microwave overall, maybe 3/4 of an inch. Premount the front of the microwave to a strip of 3/4 inch plywood that has holes that line up with the hoods bolts. Have other bolts going upward from the strip to an area of the upper cabinet that is not in the way of the cabinet's hardware.
You would adjust the back mounting hardware the same distance downward, and you could fill the side gap and cover the front edge with appropriate molding.
If it is tight, even 1/2 inch plywood should be strong enough. Add large (fender) washers to spread the load.
I think 'bib' is giving you some good stuff. However, I'm going to go out on a limb and say that one good quarter inch bolt used with an oversized washer will provide the necessary support. (This plan understand that the OTR cabinet is quality built.) The back plate needs to be well secured between two studs.
I would not do this. You almost certainly have options. While one bolt may support the weight, the chassis will have a tendency to flex and distort. This could result in visible warping along the top, movement and noise, or worse, misalignment of the door.
A better question would be how you can overcome the problem, not whether you can pretend it doesn't exist. Post some photos.