In many comic storylines, Hela is depicted as a powerful foe, but Thor often holds his own and can match or surpass her strength depending on the circumstances.
Yes. The breaking of Thor's hammer in the comics is part of complex and dramatic storylines. It can happen for various reasons related to the plot and character arcs.
He might gain new powers or abilities. Since the hammer is a powerful magical weapon, lifting it could unlock some latent potential within Peter. Maybe he can now control lightning like Thor, or he has enhanced strength and durability.
Fanfiction often takes creative liberties. It could be that there is a cross - over event that changes the nature of Thor's hammer's worthiness test. Maybe a powerful magical entity in the Harry Potter universe meddles with the hammer's magic. Harry Potter has shown great moral fiber throughout his adventures. His ability to love deeply, his sense of justice, and his unwavering determination against evil could be seen as the 'worthiness' factor by the hammer's magic, allowing him to lift it.