To give one a view of the tree of lack of communication that keeps the plot going: Aqua immediately believes Maleficent's claim that Terra removed Aurora's heart, even though the person telling her that is, well, Maleficentnote that was only very partially true. Terra, we should note, believes in just about every Disney villain he comes across (save one or two exceptions), and has his journey driven by the words of Master Xehanort. You could say he isn't guilty of trusting Xehanort, and that is true, since Eraqus didn't give him any reason to not do so... but he should have; Eraqus for whatever reason never seems to mention that Xehanort kind of tried to commit genocide for shits and giggles, instead opting for presenting him to his pupils as a righteous, good Master. The plot moves along because the good guys never, ever seem to stop and tell each other why they are doing what they're doing, or what have they found out about their particular quests, which increasingly accumulates doubt and grievances among them. Predictably, it doesn't end well for any of them.
It's not the first time in the series either. The entire plot of Kingdom Hearts: Chain of Memories happens because Sora and Co. decide to follow a ominous guy in a black coat that might as well have EVIL written across his chest into an creepy castle, and once they realize their memories are being screwed with but before any real brainwashing happens they decide to keep going instead of just leaving. By the time Sora gets halfway up the castle he's too brainwashed to even consider going back.
Kingdom Hearts 3D: Dream Drop Distance isn't immune to this either. Yen Sid tells Riku and Sora that they haven't had any formal training and thus are not as strong as they could be. Fair enough; but then he strips them of their experience, and sends them on their way to learn through experience...exactly as they had done before. Sora himself points this out roughly three to five minutes into the plot and that compared to their feats, the Mark of Mastery is at this point just a formality. Which means that there is no need for them to take this exam unlike in Birth By Sleep where the master was trusted with a few secrets about things. At one point, Yen Sid feels Xehanort's manipulations and can't do anything to stop the exam. This leads to Sora's heart being broken by Xehanort in the process. All of which culminates in Riku becoming a keyblade master while Sora gets screwed over; which to viewers felt like Sora was disqualified on an exam because some other kid (Xehanort) screwed him.
Though in Yen Sid's defence, it is stated that awakening the sleeping worlds would grant a power to help stop Xehanort which does show up as a plot point in the end. Turning it into a test, in a way, really was a formality.
Almost all the tension and conflicts of Kingdom Hearts: 358/2 Days wouldn't have happened if the trio of Roxas, Axel, and Xion actually communicated with each other. Especially the latter two: Axel withholds information from Roxas and Xion because he thinks that it is "for their own good". Xion also keeps the secrets she learned from both of them to the point that they actually have to pursue her. Ultimately their inability to communicate causes their bonds and friendship to fall apart, leading to the series' first Downer Ending, and one of the reasons Roxas gets kidnapped and placed into the Twilight Town Lotus-Eater Machine.