Thanks for the detailed explanation. It's difficult to tell what can be wrong by the screenshot. The number of objects in the family should not matter!
Try changing events like this:
Also check enemies collision polygons and origin image point - they should be symmetrical. Otherwise when the sprite is mirrored it can collide with the wall.
Randomizing enemies speed should be done on start of the layout, or on changing direction. If you do it on every tick (like on your screenshot), it will not work.
If after these changes there is still a problem with move variable, please post your project file.