even be able to remove her own Cutie Mark if she wanted to?

Suppose she wanted to Equalize herself, even just temporarily to assure the other ponies that she's trying to go along with it as well. She removes her Cutie Mark, sticks it in a jar or in the cave, then lives on without a Cutie Mark (at least until she needs it again to equalize another pony). Or she could sneak in when nopony is looking and reclaim her mark and go back to hiding it.

Would that work, or would she attempt to remove her mark only for the spell to fail due to needing her Mark to successfully cast it?

*edit* I'm stupid and forgot Starlight Glimmer was only faking her cutie mark being the Equal Mark.

It's hard to say, can unicorns cast magic spells outside their specialty? I know they can all do telekinesis, but beyond that?

The Cutie Mark Removal is probably a spell unique to her, a signature spell if you will. I would be willing to bet her original talent is bringing out the best of ponies around her, helping them find their purpose.

4270724

Magical ability isn't all that explored in the show, and it makes sense for unicorns to hyper-specialize in their own talents. After all, why learn to do a cooking spell, when there's a pony nearby who's better at cooking than anyone else? Rarity and Trixie both showed illusion spells in the past, but other than that, Unicorn Magic isn't used all that often.