Summersong
Princess of Ducks
Removed cutie marks were returned easily by locating them and freeing them from their confines. No special talent was needed to put them back on the right ponies. The ponies who’d had their marks removed didn’t have any mystical force preventing them from seeking out their marks, either.
This means that Starlight could easily have removed her own mark, since she could have put it back on when she went to “get the staff” to remove someone else’s mark, and then taken it back off pretty much whenever she had a free moment.
Of course, if simple reflexivity made it impossible for Starlight to remove her own mark (like trying to cut off both your own hands), then none of this really matters.
This means that Starlight could easily have removed her own mark, since she could have put it back on when she went to “get the staff” to remove someone else’s mark, and then taken it back off pretty much whenever she had a free moment.
Of course, if simple reflexivity made it impossible for Starlight to remove her own mark (like trying to cut off both your own hands), then none of this really matters.