Regarding who, why not any Biden family member or campaign person or donor talking to anyone in China/Ukraine. Why limit it? Might want to go after Pelosi as well, since she may be the person they want to be president after Biden resigns (and the VP resigns).
Regarding unregistered agents...so far only R's have been punished for being unregistered, not the D's. We discussed that a year or so ago. But, that's the point of this. Turnabout is fair play.
Although, I don't think the R's are as good at this unethical game. Maybe Trump is as good as the D's, but that really remains to be seen. I doubt it.