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.
Their pirate friends would probably be both excited and a bit clueless. Jack Sparrow might try to give the baby some 'unique' gifts like a small piece of treasure or a strange trinket. Gibbs would offer practical advice on how to keep the baby safe on a ship if they ever take it to sea.
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.
He would probably be quite amused by some of his past antics. For example, his constant pranks on Iruka-sensei. But at the same time, he'd also see the pain he carried from not having his parents and realize how that influenced his actions. He might react with a bit of regret for some of the stupid things he did out of loneliness, but also be glad that those experiences made him who he is today.
She likely responds with anger. Elizabeth values her independence and freedom to choose. If Mr Darcy forces her, she may lash out at him verbally, stating that his actions are unjust and against her principles. Given her sharp wit, she would use her words to make him understand that she is not a possession to be claimed through force.
If we consider the general 'Elizabeth story', Elizabeth is likely the central figure. Her significance could be that she drives the plot forward. For example, if it's a story of her journey, her decisions and actions would be what the story is built around.
The pure - blood families would be the most resistant. They would see Queen Elizabeth as an outsider and someone who doesn't understand the 'pure' wizarding ways. The Malfoys, for example, would try to undermine her at every turn. But the more progressive wizarding families, like the Weasleys, would welcome her. They would see her as a chance to break down the barriers between Muggles and wizards. And the students at Hogwarts would be divided at first, but as they see the positive changes she brings, more and more of them would support her.
In the past, their relationship could develop in complex ways. At first, they might be a bit lost and cling to each other for a sense of familiarity. As they get involved in the events of the past, they could start to see each other in new lights. Maybe Elizabeth discovers a more heroic side of Merlin as he uses his magic to protect them. And Merlin could see Elizabeth's adaptability and courage as she navigates the strange social rules of the past. This could lead to a deeper emotional connection between them.
Well, some past gods might be amused. They'd see these modern interpretations of their stories and characters in the Percy Jackson fanfiction as a strange yet interesting new form of worship or at least attention. For example, Zeus might be a bit irked at some of the liberties taken with his character, but also secretly flattered that people are still so interested in him.
Captain America would probably be very reflective. He might see his past self as a reminder of his long journey and how much he has changed. He could try to offer some words of wisdom to his younger self.