Great question Alia. Unfortunately it is still somewhat uncharted territory comparing CO2 extracts and aqueous extracts not to mention the myriad of other questions we all have on the chemical composition / effects / strains /extractions / etc. of Kava. Luckily more and more research is being compiled everyday with top universities taking more interest in the root and huge strides being made just over the last year. Some other members are probably more knowledgeable / qualified to answer this than myself but I will give it a shot.
Firstly the article you referenced is indeed a great find by GHK. A crazy amount of compiled information on kava and also solvent based extracts. From what I take away, some of the concern regarding solvent extracts resides around the actual compostion of the kavalactones changing with extraction, the concentration of the suspected harmful chalcones (mainly FKB right now), and the lack of extraction of some of the beneficial parts as you mention. What I have found through the limited research available is CO2 extraction has been the most complete method to date, leaving as little damage as possible to the whole spectrum of other constituents in kava root (or any other organic material used) and typically does not change the kavalactone profile. More interestingly though, with CO2, being able to modify the pressure & temperature with extraction, you can selectively target what you do and don't want in the resulting extract. I.E. The most prominent kavalactones have molecular weights around 230-260 g/mol and with FKB having a molecular weight of 284 g/mol, you could hypothetically leave out the potentially harmful constituents and only extract the good stuff.
All in all though it is my position that no matter how complete and innovative you can make an extract, it still only compliments
traditional kava use. They can be great methods of introducing newcomers to the benefits of kava without an off putting taste. They can also be a great on-the-go product or for times you don't feel like breaking out the tanoa and muslin bag. That is why we scour the globe to source great kava root varieties as well. Hope that helps?