Acceleration is produced by a net force on an object and is directly proportional to the magnitude of the force, in the same direction as the force, and is inversely proportional to the mass of the object.
Acceleration = Force/mass (a = F/m)
Indeed. But, and this is the point you are repeatedly failing to understand, the object pushes DOWN on the inclined plane BECAUSE (under UA) the plane pushes UPWARDS on the object.
Newton's third law.
https://www.physicsclassroom.com/class/newtlaws/Lesson-4/Newton-s-Third-LawThe only difference under a gravitational field is that gravity causes the ball to push down on the plane, that causes the plane to push back up on the ball.
The cause of the force is different but the action and reaction forces are equivalent. That is the entire premise of the equivalence principle.
In either case the object pushes down on the plane so it goes down the slope, assuming the force down the incline is greater than gravity acting up it.
You ARE wrong about this, you just don't understand all this as well as you think you do. It's worth noting that literally everyone - FE and RE, including Einstein - is telling you than you're wrong.
So...you know, it's probably you, not everyone else.