Well, the speed of light isn't the speed of anything with mass. The rest is only indirectly connected to that. The collision did already happen, from our frame

Well, the speed of light isn't the speed of anything with mass. The rest is only indirectly connected to that. The collision did already happen, from our frame of reference. Its light arriving to us will be just a confirmation. From some other frame of reference (one moving very fast in the line that joins Earth and the collision), it might not have happened yet.