Indeed. If Citizendium are to be sued over this, it would be a class action suit brought by a large number of Wikipedians.
Ahahaha, no it wouldn't have to be. Any individual whose copyright in their written text was violated would have a cause for action. Imagine 1000 separate DMCA notices to a violator and their host.
It wouldn't *have* to be, no, but surely the most practical method is a class action suit?
It's unlikely to happen that way, I would hope - CZ's intentions aren't to do evil. I have a CZ login and I last used it to fix an image attribution (sufficient for the terms of both GFDL and CC-by 1.0). It depends if those who appear primarily motivated by messing with Wikipedia can be held back from massive copyright violation to that end.
However, it seems Larry Sanger falls in that category, and holding him back may be rather challenging.
Statutory damages under US law require registering one's copyright. That can be done diff by diff, much as IBM registers each of its Linux kernel contributions. Note that people outside the US, like me, can still send valid DMCA notices to US hosting entities.
Could you really claim that copying an article in which you made 2 back-to-back edits counts as violating the copyright on two works? That seems rather arbitrary.