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As a thought experiment, suppose we bombard the black hole with antimatter thus annihilating the mass? Could that make the black hole white again? At the very least could it make the radius of the event horizon decrease putting Alice outside the horizon once again?

There could be a couple of flaws.

The radiation produced by annihilation might gravitate as much as the mass did. Then the radius of the event horizon would increase.

It could also be true that a particle falling into a black hole can never catch up with another particle that fell in earlier. In that case the antimatter would never meet the matter. No annihilation would occur and the radius of the event horizon would increase.