Re: "This is fantastic news ... FOR SCIENCE!!!"
"But if the crash down causes a trajectory change and it heads earthwards!!!!"
Have you considered just how unlikely that is?
The comet's orbit is outside the Earth's orbit at perihelion. So you would need to add significant delta-v to get Rosetta to cross the Earth's orbit. The crash won't do that. I have just given up trying to calculate the chance of Rosetta hitting the Earth if you managed to create a sufficiently big deflection with the amount of fuel that's left, but it is definitely well under 1 in 10000. And then of course there is the direction of impact and the time in atmosphere to determine whether or not it is going to burn up. Rosetta doesn't have a RTG, so no worries there.
Or in simpler language, it's a very long way away travelling in the wrong direction and there are a lot of far more important things to worry about.