A:Answer As long as you're not actually blocking the vent intakes or top exhaust, in theory you could do this. In theory. but...
A unit designed as a built-in will work out better.
And someone, someday, *is* going to fire up that 500cfm blower. At full speed. Possibly for a long time, like, while the kitchen fills with smoke and they panic over whatever it is they're cooking. A safe installation has to build in readiness for that.
If you were going to do this in a wall-flush mount, I'd leave at least 4" open space under the unit to access the filter elements, and 1" open over the top. In a 12-15" depth cabinet stack (3-6" of the microwave protruding) the top could be installed flush without blocking the microwave's vents, and the bottom would need just 3" clearance to service the filter elements.