It's generally hard to get stars to collide with black holes since even the very largest black holes are still very small relative to the distances between stars. There's just too much angular momentum that needs to be removed. (Which is not to say that it can't be done -- it can, and there are various mechanisms for doing so, but it's very difficult to do for a substantial fraction of a galaxy's mass.) Even if this occurred, it's nevertheless surprising that it happened in this particular galaxy, and didn't occur in pretty much every other one.
Far from the event horizon, a black hole acts like a point mass just like every other star. If, for instance, the Sun were to immediately turn into a black hole, the Earth would continue to orbit it as if there had been no change.
Assuming it kept the same mass. If it had a lot more mass, the Earth's orbit would be identical to it's orbit around a star with that much more mass. The fact that an object is a black hole only becomes relevant when something passes the event horizon. Outside the event horizon, it acts like any other object with the same mass.
Yes. The radius of the "event horizon," which is the spherical boundary that serves as the point of no return, grows as it swallows more mass. For a Schwarzschild black hole (which doesn't rotate and has no charge), the event horizon radius is 2GM/(c^2), where c is the speed of light, M is the mass of the black hole, and G is the gravitational constant.
The singularity at the center doesn't grow. But a black hole's "size" generally refers to the radius of its event horizon.
I've heard a black hole's singularity defined as 'infinitely dense and infinitely small'; I don't think such a thing can be said to 'grow'; however, as the density increases I think that the Event Horizon expands outward increasing the diameter of 'black' so the answer to your question may be yes.
Yes. Formally, a singularity is just a "hole" in spacetime. As you increase the mass of the black hole, the singularity itself doesn't change, but the curvature of the spacetime around it becomes stronger.
My (poor) understanding is that the "size" of a black hole refers to the size of its event horizon, which is directly related to its mass (more mass -> larger event horizon). That mass is still compacted at the center in a single point of zero (?) size.
http://accelerating.org/articles/transcensionhypothesis.html
http://www.youtube.com/watch?v=nQOyJUDTKdM