They're not faster than C, but they could outrun a beam of light if there was some stuff hanging out in space and raising the index of refraction. In relativity the C limit is geometric, in the same way that "you can't make an apple bigger by rotating it around in your hands" is geometric.

Not only can, but do.

The intergalactic medium (IGM) is very diffuse, but not pure vacuum. The difference in arrival time of light and gravitational waves from an event billions of light years distant could help measure the density of the IGM.

Incidentally, while it is very cold, the IGM is plasma, so obeys plasma fluid-dynamic laws, which is why the clouds seen at the ends of Active Galactic Nucleus jets, millions of light years across, can be visibly turbulent.

No