The wizarding families would be divided in their reactions. Some families, like the Weasleys, would be ready to fight against Riddle immediately. They would gather their resources and join forces with the Order of the Phoenix if it still exists. Other families, who might have been sympathetic to the dark arts in the past, could be more hesitant, not sure whether to side with Riddle or the rest of the wizarding world.
One possible reason could be a dark magic spell gone wrong. Maybe some wizard tried to experiment with a time - altering spell and accidentally reversed whatever transformation Voldemort (Riddle) had undergone. Another reason might be related to some ancient magic artifact. If Riddle's essence was somehow bound to an object, and that object was tampered with, it could bring him back to his old self.
The self - insert could change Tom Riddle's fate by showing him true friendship. In the original story, Tom Riddle was a lonely boy who turned to dark magic. If the self - insert befriends him and is always there for him, it might make him less likely to seek power through dark means.
Some might be confused. His friends could initially be shocked and try to understand why he made such a drastic change. They might worry about him distancing himself from them.
Well, his return would bring back a lot of memories. Some of his friends might have moved on in different ways, so there could be some initial awkwardness. But overall, they would be glad to have him back. They might also look to him for guidance in new problems that the wizarding world is facing, since he has so much experience dealing with danger.
A good old fanfiction could be the one where Tom Riddle's relationship with the Chamber of Secrets is explored in more detail. It might show how he found out about it and his early experiments with the Basilisk. This could give a new perspective on the events in the original books.
An old Harry Potter fanfiction about Tom Riddle could be about his time as a young wizard trying to gain followers. He was a very intelligent and manipulative character. Fanfictions might explore how he used his charm and intelligence to persuade other students to his way of thinking. Maybe he starts a secret club where they study dark magic together, which is a precursor to his later Death Eaters.
Tom Riddle might be very suspicious at first. He is a very intelligent and cautious person. He would likely view time travelers as a threat to his plans or as a strange new phenomenon that he wants to control. So, he could start by trying to figure out their motives and how they managed to travel through time.
Maybe he discovers some legal loophole in the wizarding world's financial system that allows him to reclaim his inheritance. For example, if there was some improper handling of his family's vault by the goblins at Gringotts, he could use that as a basis to demand his money back.
His character development makes him more confident. So at the trial, he can firmly defend himself without hesitation.
In this fanfiction scenario, Harry could be initially stunned. He has to come to terms with being a father and also deal with the potential implications of the Tom Riddle connection. He might be very protective right away, making sure his kids are safe from any harm that could be related to the dark past of Tom Riddle. His reaction could also be shaped by how much he has changed since his own battles with Voldemort (who was once Tom Riddle), and he'd be determined not to let his kids face the same kind of danger.