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A lot of the questions thrown up in this thread are indeed some of MANY that astronomers would like answered by the Rosetta mission.
Ice on comets isn't completely understood, and it's a lot different from the ice in your freezer, this ice is thought to be amorphous which has no crystal lattice structure and must be bloody hard!
Not all comets are a 50/50 mix of rock and ice, and the jets are formed through active regions on the nucleus.
I would have to re-read some things on the subject of the make up of cometary nuclei, but with the data astronomers have at present the dirty snowball model is the best fit so far.
I will be interested to see what the Rosetta mission pulls up!
As for the Deep Impact mission, they were expecting a football sized area to be excavated by the energy released upon vaporisation of the impacting projectile.
From memory the copper slug was 500kgs and about the size of a washing machine!
The amount of material being blasted out is also governed by the density of the comet.
The double flash could be explained, i would imagine, by the vaporisation of the projectile firstly, and the shockwave from the rebounding surface of the comet which threw material from the nucleus afterward.
The camera used for the impact investigation was only supposed to view the crater in the aftermath of the impact, they should organised it to come back a bit later when the dust died down! lol
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