Imagine, just for a moment, that when they take the portkey, instead of an Avada Kedavra, it’s a Stupefy that hits him. Or that the AK misses him by an inch, hits a grave instead, knocks him out for a second.
Imagine that everybody forgets about the Hufflepuff boy out cold on the floor, because they are so intent on resurrecting Voldemort. Peter forgets as he ties Harry to that statue. Voldemort forgets as he is dumped into a cauldron full of flesh and bone and blood. And every death eater that comes sooner or later, well, no one tells them about the boy either - there are more pressing concerns.
However, Harry doesn’t forget. Because Harry has been in that sort of situation since he was eleven. He’s used to looking out for others, by now. Hermione and the Troll, Ron on the chess game, Ginny in the Chamber, Sirius and Hagrid and even Buckbeak- Harry always looks out for everyone, and never forgets about anyone, even if they are not really his friends.
So while he stares in horror, while he’s powerless and sees his greatest foe come back to life, a tiny part of his mind is screaming at him to check on Cedric, to get them out of here. Both. Alive.
Now let’s say that the ceremony, and the Death Eater meeting after the resurrection takes time. Lots of it. Let’s say that Voldemort, being the drama queen he obviously is, takes his time, and enjoys every single second of attention he gets from his followers and that Potter brat.
Let’s say he takes enough time for Cedric to come back to consciousness.
He awakes, lying in the grass and dirt, surrounded by bits of stone, his head aching and confused. The cup is laying about, not too far from him, and he could take it to go back but- he’s a Hufflepuff. He’s loyal. He doesn’t forget either, and that’s why, even if he’s confused about why or how he’s here, he doesn’t take the cup and goes searching for Harry.
Now, the tournament is a vicious thing, isn’t it ? Who’s to say to poor confused Cedric that this is not one more, secret, task ?
So Cedric goes looking, wand in hand, ready to fight, because he’s a Hogwart champion - and really, a Graveyard ? That’s creepy. And because he’s on his guard, and he’s moving around silently, no one notices him creeping behind one of the graves. No one notices the Hufflepuff boy, his horrified expression, and his frantic gaze as he slowly understands that no, that wasn’t a task, and that wasn’t a dream either.
Maybe not even Harry, or maybe he does, but that’s not the important thing.
The important thing is that being in Hufflepuff doesn’t make you stupid at all. The important thing is that Cedric is a champion, and smart, and a quick thinker and a hard worker.
The important thing is that Cedric thinks fast, and casts an ‘Accio’ on the cup as he runs towards Harry while he duels Voldemort.
He breaks through the crowd of amazed and struck Death Eaters, catches Harry’s arm with one hand, and with Seeker reflexes, catches the cup with the other.
Cedric lives, and both Harry and him go back to Hogwarts, terrified, bloody, and flinching away from the sudden noise coming from the public. They both live, and thus no one notices that something is amiss immediately, no one sees their wild glances around - as if someone was still out to kill them. The public cheers, and sings the victory of both Hogwart’s champions, and they are suddenly hugged by their families - the Diggorys and Weasleys.
No one notices, and that’s why when the noise dies down, and someone casts a sonorus on them to ask them how they feel about that victory, everyone hears them say, in a still disbelieving and trembling voice.
Obviously, everyone is confused, but they start talking, a bit over each other really, but they are in shock - and they say he’s back, Voldemort’s back, and he took my blood, and we were in a graveyard, and I was knocked out, missed most of the ritual, but it was him, yeah, and there were Death Eaters, in a circle, torturing Harry, horrible, had to get away, he’s back, he’s back.
And that’s when the people notice their faces, the blood, Harry twitching fingers - cruciatus - and their wands still clenched in their fingers, as if ready to attack anyone on sight.
This time, though, Harry doesn’t get ushered away by fake-Moody - because Cedric still has a hand gripping his arm, and wont let go for the world. He tells Dumbledore, and their families, though, when the Headmaster asks them to talk “More calmly and clearly, please, young men” at the Infirmary. Barty Crouch Jr is still apprehended, and the real Moody discovered, and it puts their incredible tale in a new, horrific and real, light.
Imagine if Cedric Diggory lived.
Two witnesses of His return. One is Harry Potter, Hero and Saviour of the Wizarding world. The second is beloved Hufflepuff Prefect Cedric Diggory, Hogwarts Champion. Even if people didn’t believe the first, they would believe the second, and vice versa.
Obviously, the ministry doesn’t take it well, but Amos Diggory and the Weasleys, and Dumbledore make a move together. Susan Bones helps her fellow Hufflepuff by contacting her aunt. Together, they get memory evidence - and they even agree on submitting to truth serum.
Because if Harry alone couldn’t do it - or had no idea he could - Cedric is there, and his father works at the Ministry, and he’s a seventh year. He knows more, and he has people ready to help him - and if he asks them, to help Harry Potter.
Sure, the ministry would try to get all this under the rug, but they couldn’t. Because Weasleys, and Diggorys, and Dumbledore, and Bones, and even Longbottom and soon every name that has a contact in Hogwarts - except some of the Death Eaters - are pushing for the truth to get out, and with a bit of blackmail, Rita helps - and this time, the Daily Prophet can’t repress all of them.
Imagine if Cedric Diggory lived, and how the war would have turned.