Brock, is that really a picture of broccoli with Lee's hair posted on top of it? Wow!
However, as far as the Zetsu explanation and that not being the real Neji...it's doubtful. They would have to have at least shown a situation where he would be switched. This isn't Kubo writing.
Actually it seems believable, the situation can be shown in a flashback anyway...
Besides, is it just me or does Neji's face just seem somehow wrong all the chapter? Couldn't help but think that something's not right and then wham, it turns out Neji's on rampage.
Either way it'll either be Zetsu substitute or just Itachi's genjutsu, but I'd go simply with Neji's byakugan meeting "eye to eye" with Itachi's sharingan by mistake.
Although that'd mean that all Hyuuga members who use their byakugan close enough to find itachi are now screwed, so it might be a substitute after all.