Replacing overtightened molly bolts
A French Cleat is a piece of wood (or metal) that runs most or all of the width of the object to be hung. It is installed on the surface of the wall, sized to hide behind the object being hung. Unless you're hanging a mirror that's less than 16&quot; wide and the only place you can hang it happens to be exactly between two studs, some place on that cleat should cross a stud. If you want to hang it here for aesthetic reasons, then you may want to reconsider the aesthetics for practicality and safety and move the mirror an inch or two one way or the other to find a stud. If, of course, the mirror is more than 16&quot; wide, it should cross a stud somewhere. (Note: assumption of US building codes and standard, modern building methodology. If you live outside the US or your house was built prior to 1900, all bets are off.)
Put a proper screw/bolt into the stud, then use mollys/toggle bolts/whatever to prevent the ends from spinning around the central support point.
One good screw/bolt should be able to support your 12 Kg load, so any additional attachment points are bonus weight bearing support in addition to their &quot;anti-tip&quot; job.