If the roof is built with trusses, the wall may not be load bearing. You would need to check in the attic to see if it's stick framed or not.

Most new microwaves have an exhaust vent that can go out the back of the unit or the top. , this would allow you to stay in the cabinet and go straight up to the attic. You lose some storage room in the cabinet.

If you do vent to the attic I would recommend extending the pipe to a roof or gable vent, you can pump a lot of moist air up there and that could be an issue.