Can I support the front of a microwave with only one of three bolts?
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.