Dobby terrorizes Neville to stay out of Hogwarts. The Potters would no doubt react to this, perhaps charming the house to keep Dobby out, or simply convincing Neville to continue with his education. (Stay in school, kids!)
Now comes the issue of school starting again. If the boys happened to be visiting their friend Ron during the summer vacation, then Dobby would close the platform to all three of them. Ron would offer his father's flying car, and they would arrive mostly safe at Hogwarts. If the Potters bring the boys to the platform, then they would have no doubt waited for their children to enter first before themselves entering. This means that Lily and James would be blocked out of the platform as well, and would have handled the situation. If the Potters for some reason did rush ahead of Neville and Potter, then they would have immediately noticed the absence of the only children they were watching. Even if the Potters managed to be particularly unobservant, Harry and Neville would have been forced to think about their actions, no doubt landing on the owl option. (It's here we note that Hedwig would not be known to the readers in this world, because she was a gift from Hagrid to Harry before his first year. Sorry, Hedwig.) There's always the fourth option that if the boys didn't think of sending a letter, that Matt Smith would pop his head out of a blue police call box and offer them a ride. No matter the option, Neville, Harry, and Ron find themselves at Hogwarts School of Witchcraft and Wizardry, and aren't expelled.
While watching one of Harry's quidditch matches, Neville is concussed by a very rogue bludger. No doubt when Lockhart tries to help him, the incompetent buffoon would kill our Chosen One. DEAD. If Lockhart was kept away from the injured boy, then Neville ends up in the hospital wing. There Neville hears the voice of the basilisk.
As Hermione tells us, hearing voices isn't even normal in the wizarding world. There's a chance that Neville would be too embarrassed to mention it to his friends. This would result in Ginny's death and Voldemort's return. When Voldemort returns, he would kill Neville. DEAD. If Neville mentions the voices to his friends, the plot would carry out as it does in Rowling's novel. Except, it's worth noting, that Neville would or would not accompany Harry and Ron on their Draco spying spree. This doesn't affect the plot too much on either side, so let us skip to the Ginny's disappearance.
If the boys had not arrived to the school by flying car, then Ron's wand is fully functional. When Lockhart snitches Ron's wand and performs a memory spell, it would work correctly and the cave would not collapse. If Lockhart had erased Neville's memory first, then the world is screwed and Voldy makes a return. Voldemort would then kill Neville, with or without his memory. DEAD. If Lockhart has erased Ron or Harry's memory, then the remaining two boys would get Lockhart back under control. Lockhart would be assigned babysitter to the memory-lost child (and most likely tied up). Neville and the remaining boy would continue forward.
If the boys do arrive by flying car and Ron's wand does backfire, then Lockhart and Ron are trapped on the other side of a collapsed tunnel. There's always the chance that Harry is trapped with Ron too, in which case Neville carries on alone. Fawkes would still bring the Sorting Hat, and Neville would still slay the beast. We assume that Neville would injure the journal, but would most likely use the sword of Gryffindor instead. As goblin metal absorbs things that make it stronger, this would have eradicated Riddle and revived Ginerva. If Neville had the misfortune to attack Riddle instead, he would have failed and been killed, along with Ginny. DEAD.
If Neville and Harry proceeded together, then Harry would compliment Dumbledore and summon Fawkes with the Sorting Hat. Neville pulls out the sword and stabs the basilisk. While Riddle brags to Neville about his return, Harry impales the journal. They defeat Voldemort and save Ginny, allowing Creager and me to get on with our lives.
(Two notes. First, would Ginny still be crushing on Harry or would she instead set her sights on Neville? Second, the Hogwarts plumbing has a rotting snake in it. Someone should check that out.)