OK I can see where your coming from.....But if I were to tear a building down brick by brick, would it be correct to say that the individual bricks weigh less than the entire building?? Of course.....
But would it be correct to say that the combined mass of all the bricks is less than that of the building?? (Ok ok forget about funiture and electrical wiring...etc.)
Can you see where I`m going with this?? Now the energy release of a matter/anti-matter reaction is way more complex than a ton of bricks, and certainly more spectacular (and violent) than tearing down a building. But the point is, if you add up all the individual energy amounts contained by each photon, the resulting mass (this is where E=MC^2 comes into play) should equal the initial mass of the reactants. Once again energy is conserved.
Now I can`t tell you if the matter will even react with the antimatter within the confines of a blackhole. I don`t think anybody can tell you that, or even if the matter/antimatter is even still considered matter or retains its properties.
All I can gaurantee you, reation or not, is that the mass of the blackhole will increase if you add any mass period, be it anti-matter or not. The released energy, should there be a reaction, doesn`t get to escape. It`s trapped forever (lets ignore the hawking effect for purposes of making the above point).
And finaly I`ll leave your last question to bladesinger to answer. He`s pretty smart at this physics stuff and I couldn`t come up with an answer. All I can say is that the only way an event horizon will shrink is due to the hawking effect which for large blackholes happens VERY (and I mean VERY) gradualy. I`m assuming that you`re talking about the photons on the interior side of the event horizon.