I can try to explain it different.
Create an Object Sprite - Goomba
Give Goomba the Behavior Platform
Uncheck 'Default Controls' this will remove your ability to control them with the keyboard arrows
Now you need Code to Simulate Move Right if they are not mirrored and Move left if they are Mirrored
Easy route to make them turn is to have 2 object s - one called 'forceMirror' another called 'forceNoMirror'. Set these both as invisible and place a 'forceMirror' on all right edges of platforms and the 'forceNoMirror' on all left edges. Now put a check for each Goomba, if overlapping 'forceMirror' Set Goomba Mirror True and another for each Goomba if overlapping 'forceNoMirror' set Goomba Mirror False.