I don't suppose the remaining 7 kilos of thruster fuel had anything to do with a larger than expected result?
We'll see.
No, a drop in the bucket. Kinetic energy is capable of not only vaporizing both the spacecraft and some of the body of the comet, but of ionizing a portion of the resulting gas. The initial flash was due to the ionization effect. Totally expected. The linear features of the dust/gas plume are an effect of near zero gravity, also totally expected.