Really? The men of the CCG are sooooo gullible, not even Arima could kill Kaneki, they thi...

That's not how it works and you're not thinking very far if that's what you think is going on.

Rumors can reach far very quickly, it's likely that pretty much everyone that survived the clowns attack have human blood on their hands and know it. That's why there was talk of their morale. Pretty much all the CCG investigators must feel like murderers. Guilty and ashamed.
That's a weakened mental state and actually probably exactly where Furuta wanted them as he took over the director position.
Now what Furuta did was put on a show. And the sole purpose of that was to let the rest of the CCG lie to themselves. It's much more easy for them to pretend they don't know, go along with the public image and the official made up version, then look at themselves and know they're no better than the ghouls they hate now.

Furuta had them at their wits end, then created a convenient lie so everyone in the CCG just have to cover their ears and eyes and pretend they did capture Haise and this is all a big success.

People who stronger psyche like Ui or Urie obviously already know they're been killers and don't need that sort of lie to keep them going. But they're realizing what is going on and if the CCG lie to itself and it's purpose, then yeah it's pretty much gonna throw morale out the window (the children soldiers trope, so we're already there) and pretty much walk to their end now.

So no, these investigators aren't gullible. They're humans and in the worst position ever with the worst manipulative shit as their new chief. They're just screwed big time and doing whatever they can to keep it together and live on.

Its not like i am not thinking too far... they always had have blood in his hands, they are losing everything they believe it was right, they are just hiding and pretending that everything will go the way they think its right... but the fact that they killed people and now that they think they killed Kaneki maybe it is conected, but pretending that the dead of Kaneki is a win for them is what make them gullible, i am talking beyond the word "gullible", they want to be gullible to remend all the situation, that it is what made them gullible....
Well even if you don't understand what i am saying, i can understand what the situation is and what they are feeling, and what i wanted to comment up there... it was just a general comment :)