I thought the possibility raised by the expert witness of it having been cracked by a glass thrown at it sounded convincing to me. BUT as you noted, how would KR have known it was cracked then? If JO had thrown his glass, possibly in anger, at her vehicle, and it cracked her tail light, she could easily have heard/seen that, and been aware that the noise she heard was her tail light breaking. But if she knew that, why wouldn't she have told anybody this? Might be explained by her being drunk and possibly blacked out that part. Other than that, no explanation I can think of, which would mean there must be some other answer as to how it was cracked.
I do think the expert could be right about some things he said, but still be wrong about that part. And IIRC, he did say that was just his opinion that she wouldn't have cracked it backing into JO's vehicle. I don't think they studied that aspect of the case thoroughly either, so it's possible he would have a different opinion had they done so.
Does raise more questions though, I agree.