Maybe some old pure - blood families who want to regain power. They view Harry as a symbol of the new order that they oppose, and Snape as a tool they can use. By forcing this marriage, they hope to control both of them and use their combined influence for their own dark purposes.
There could be a new, previously unknown dark wizard or witch. This character might have a vendetta against Harry and Snape for different reasons. Forcing them to marry could be part of a larger scheme to get revenge on them and also to undermine the stability of Hogwarts and the wizarding community as a whole.
Perhaps there could be a magical object or spell involved that is causing this forced marriage. Harry and Snape might have to work together to find and break this spell. Along the way, they could learn more about each other's pasts and secrets, which could change their relationship from one of enmity to something more complex.
One common theme is power struggle. Since Snape has his own power and influence in the wizarding world, and Harry is the 'Chosen One', a forced marriage between them could be a way for different factions to gain control. Another theme could be identity crisis. Harry would surely struggle with his identity as a Gryffindor hero being married to Snape, a former Death Eater and Slytherin.
Yes, there are some. You can start by looking on fanfiction platforms like Archive of Our Own. However, the idea of Harry being forced to marry Snape is quite an unusual and often dark or complex concept in fanfiction. Some stories might explore this in the context of a magically - bound contract or some sort of political intrigue within the wizarding world.
One reason could be a misfired spell by another character. For example, if a young wizard was experimenting with a transportation spell and it backfired, it could accidentally force Harry to be attached to Snape in this 'ride' situation. Another reason might be a plot by a dark wizard who wants to see chaos at Hogwarts. They figure that forcing Harry and Snape together in this strange way would disrupt the normal order of things.
Perhaps there is a political reason. Some powerful pure - blood families might be trying to strengthen their influence by uniting the Potter and Malfoy families through this marriage, even if it's by force. Another possibility is that there is a prophecy that they misinterpreted which led them to force this marriage.
A powerful and ancient magical being might force Harry to marry. This being could have its own motives that are not immediately clear. It could be related to some prophecy or an old magical law that only this being knows about. Harry would then have to figure out the nature of this being and how to persuade it to release him from the forced marriage.
Maybe it's a political move within the wizarding world. The Ministry of Magic might force this marriage to unite two powerful magical forces. Harry would then have to deal with his feelings for his friends like Hermione and Ron, who would be shocked. Snape, on the other hand, would have to adjust his view of Harry from a student to a partner. They could start to find common ground in their love for Hogwarts and magic in general.
One possible plotline could be that it's a political arrangement. Maybe there's some sort of wizarding world politics where uniting the Potter and Greengrass families is seen as beneficial for stability. Harry might initially resist but then gradually find out that Daphne has her own charm and they start to develop a relationship based on mutual understanding. For example, they could bond over their shared experiences in the wizarding world, like dealing with pure - blood prejudices.
The other Hogwarts students would have a wide range of reactions. Some might be shocked and gossip about it. Neville, for example, might be sympathetic towards Harry. Draco Malfoy could use it as an opportunity to taunt Harry, but at the same time be a bit envious of the attention this situation brings. The teachers would also have different responses. McGonagall might be worried about the impact on Harry's education and future.
Well, it's hard to say exactly without reading an actual piece of such fanfiction. But perhaps it could involve Harry being in some sort of difficult situation where he has to force Mary Snape (assuming she's an original character) to do something for the greater good, like revealing a secret that could help in the fight against dark forces. Or maybe it's a misinterpretation situation where Harry thinks Mary Snape is hiding something dangerous and he has to use force to get to the truth.