Hermione could be the one. With her vast knowledge of the wizarding laws and her sharp intellect, she might dig up some old regulations that prove Harry's right to his money. She would then fight tooth and nail in the wizarding courts or against the goblins at Gringotts to get his money back.
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.
The 'harry gets his mail back fanfiction' could start with Harry noticing that he hasn't received any mail for a while, which is strange in the wizarding world. He starts to suspect foul play. As he investigates, he finds out that there is a new spell that has been cast on the mail delivery system that is misdirecting his mail. With the help of Ron's family's connections in the wizarding world (since they are so well - known in the wizarding community), Harry is able to track down the source of the spell and reverse it, finally getting his mail back.
In this kind of fanfiction, one possible plot could be that through some magical accident or powerful spell, Harry discovers that Snape is actually his father. Maybe a time - traveling spell goes wrong and reveals long - hidden secrets. Harry then has to deal with the newfound relationship, and Snape has to come to terms with being a father figure he never expected to be.
It could be Draco Malfoy. He might be jealous of Harry getting certain important mail, like letters from Dumbledore or invitations to exclusive wizarding events. So, he uses some underhanded means to make Harry's mail disappear. Draco would think it's a funny prank at first, but when Harry starts to fight back, it turns into a more serious conflict between them.
Harry would likely become more conflicted. On one hand, he has always seen Snape as a difficult figure, and now he has to accept him as his father. His view of himself and his family would change. He might start to question his Gryffindor values as he realizes he has a Slytherin father. This could lead to a more complex and nuanced character as he tries to reconcile these two different parts of his identity.