Diane would probably be overjoyed. She's always been close to Meliodas and Elizabeth, and she would likely offer to protect them and help out in any way she can. Maybe she'd use her giant strength to build a special place for them to stay during the pregnancy.
Ban would probably be excited and start thinking about all the fun things he could teach the baby, like how to steal food without getting caught in a humorous way. King might be a bit more reserved at first but then offer to make the most beautiful crib with his powers. Diane would be super excited and offer to babysit all the time, cooing over how cute the baby will be.
Some characters might be angry if the fanfic misrepresents them completely. For instance, if a heroic character is made into a villain in the fanfic, they would surely not be happy about it.
Naruto would likely be very curious and excited. He'd probably try to befriend everyone, whether they were humans, demi - humans or monsters. His positive attitude would be a shock to some of the more cynical characters in the Overlord world. Sasuke, on the other hand, would be more guarded. He'd assess the situation and look for ways to gain power. He might see the powerful magic and different races as a means to achieve his own goals.
Yes, there are likely fanfictions out there with such a concept. Fanfiction writers often come up with creative and unexpected crossovers like this. You can search on popular fanfiction websites such as Archive of Our Own or FanFiction.net to find stories based on this idea.
The Death Eaters and their sympathizers would be terrified. Harry, as the overlord, would be a major threat to their ideology. Voldemort, if he somehow still existed in this fanfiction, would be in direct opposition to Harry. However, some of the more neutral characters, like Luna, would probably see Harry's new role as an opportunity for positive change in the wizarding world and would be happy to support him in his endeavors.
Fluder is very intrigued by Momon's presence. As a mage always seeking knowledge and power, he sees Momon as a unique entity. He first tries to observe Momon from a distance, then gradually tries to approach him. His reaction is also laced with envy to some extent, as Momon seems to possess power that Fluder may not fully understand, and he desires to have that kind of power for himself.
In fanfiction, female Meliodas often has the same fearless and strong - willed nature as the male version. However, she may also possess a more empathetic side. For example, in some stories, she's more in tune with the emotions of those around her.