((so my guess is that this is what happens... Visser 5, Hu, and Kelran strike a deal. If Kelran submits to infestation, and Hu submits to torture, they wont allow Bened to kill Hu or over-permanently damage him, and they wont infest cody.
What the Visser's thinking is, is that after Kelran is totally infested they can fully torture and kill Hu, and infest Cody anyways.
What they didnt count on is Kelran's neuro-physiology, which gives him a vastly increased line of internal defense.
So what they plan to do is keep switching his Yeerk in shifts near the end of each three day limit until his control is broken. But right near the end of the three days, the others show to save the day, and the Yeerk dies of starvation in the battle, and Hu carrys unconcious Kelran outta there.
This lets us have great torture scenes between Hu and Bened, and great moments between a Kelran whos phasing in and out of partial infestation, and Cody whos really p'oed. and Hu gets to save Kelran wich strengthens their friendship. Good?
EDIT I also have an explanation for why his infestation must be voluntary. When he flattens his ears a certain way it brings a thick piece of bone down inside his ear canal. Ordinarily the Yeerks would drill through it, but because of its density if you applied enough force to do so, you would shatter his whole braincase and kill him ))