Merlin would probably be very conflicted. On one hand, he is loyal to Arthur and Camelot, but on the other, being forced into a marriage is a huge infringement on his freedom. He might be scared about how this will change his life and his ability to use his magic secretly. Arthur, on the other hand, could be initially resistant as well. He sees Merlin as a friend and a servant, and this sudden change to a marital relationship would be strange for him.
Merlin's reaction could be one of shock and disbelief at first. He might try to find ways to get out of the situation, using his wits and magic if possible. Arthur may be more stoic in public, but privately he could be confused and a bit angry that such a decision has been made for him. He has his own ideas about his future and who he wants to be with, and this forced marriage throws all that into chaos. The other characters in Camelot, like Guinevere, would also have interesting reactions. She might be jealous or worried about how this will affect her relationship with Arthur.
One possible plot could be that Merlin has some hidden magical ability that makes him a valuable asset in a political marriage to Arthur. For example, his magic could be the key to uniting two kingdoms. Another plot might involve a magical curse that forces the marriage. A third idea could be that Merlin and Arthur are tricked into a marriage by an evil sorcerer who wants to disrupt Camelot from within.
Merlin could be heartbroken. He might retreat into himself and become more withdrawn. After all, he has a deep bond with Arthur, and this sudden break - up would be a huge shock.
Merlin would be very worried. He would probably stay by Arthur's side day and night, using his magic secretly to try and heal him. He might also search for rare herbs or magical items that could cure Arthur.
Arthur might be initially shocked and confused. He's used to Merlin being in control. But then he might find it quite hilarious and start teasing Merlin about it.
Naruto would probably be extremely confused at first. He has always seen Itachi as a complex figure, and suddenly being forced to marry him would be a huge shock. He might feel angry that he has no say in this. Itachi, on the other hand, would likely be stoic on the surface but internally be very conflicted. He has his own burdens and this unexpected situation would add to them.
Merlin might be initially shocked and heartbroken. He has been so close to Arthur, and this sudden forgetfulness would be a huge blow to him.
He might also get angry. Merlin has his own pride and after all he has done for Arthur, being insulted like this would make him furious. He could start yelling back at Arthur, listing all the times he has saved Arthur's life or the kingdom.
Merlin would likely be seen as a bit of an enigma at Hogwarts. His magic is different from the norm, so he might be a bit of an outsider at first. But his kind heart and loyalty would soon win over his classmates. Arthur would be the brave and headstrong character, perhaps a bit like a Gryffindor version of his usual self. He'd be eager to learn about combat magic and would probably be a natural leader in group projects or Quidditch teams.
Some of their friends might think it's a strange situation. Luna could see it as a very interesting and magical development, believing that there must be some deeper, cosmic reason for it. Seamus and Dean might be a bit skeptical at first but would warm up to the idea as they see Harry and Hermione making the best of the situation. The Weasley family as a whole would probably have a mix of reactions, from Molly's motherly concern to Fred and George's attempts to find the humor in it.
In BDSM fanfiction, Merlin's character might develop a deeper sense of trust. Since he is often in a submissive role, he has to trust Arthur completely. This could lead to him becoming more open and vulnerable, which is different from his usually secretive self in the original series. For example, he may share his true feelings more easily.