So, the eastbound car has already passed through the intersection, and the westbound cars are approaching the intersection. Traffic is backed up as they approach the intersection, so the cars in the near and middle lanes stop to let the guy turn, but someone else in the middle lane guns it to the 3rd lane (which at this point is a turn only lane as they approach the intersection). Is that all correct?

That would clear up a lot of my confusion. When you said the car turning was "half a block from the light" I assumed you meant half a block before getting to the light, not half a block after having passed through the light.

If it's close enough to the intersection that a dedicated turn-only lane has already started, then I would imagine that there is a double-yellow line separating the directions of traffic, which means the guy turning crossed a double yellow and is completely at fault.