Well, I don't have time to read the whole of both studies but from a very cursory look, it seems that the second article concludes that kava in an aqueous solution is safe (as aqueous solutions contain glutathione). Based on the abstract alone, I would interpret it to conclude kava actually has cytoprotective properties. The 40% cell death with glutathione w/ lactones only indicates that glutathione decreases cell death by 60% compared to lactones w/o glutathione. The test didn't even appear to be in vitro as it didn't seem to even use mammalian cells BTW.
The first study has some methodological flaws in it, IMO if they are trying to translate the safety of kava as we know it. I'll have to dig deeper into that one when I don't have exams to study for, but I wouldn't make anything out of it. The rhetoric is strong, but don't let that scare you out of the facts. You would think that a more applicable study would say "rats treated with kavain via parenteral administration" or something along those lines. Without looking through the whole thing, it sounds like they treated the livers in a way that would never happen during normal human physiological functioning.
EDIT: It actually seems that the first study actually invalidates the 2nd study (in terms of normal kava consumption in humans).