Nagisa's best friend might be really surprised at first. But then, being a true friend, would support Nagisa through this strange change. They would probably help Nagisa with things like choosing girl - appropriate clothes or dealing with any mean comments from other students.
Their other classmates like Okuda would be very sweet. She might make them some special potions or concoctions as wedding gifts, something that symbolizes good luck and a long - lasting marriage. And she would be all shy and cute about presenting it to them.
Kayano would be completely devastated. Since she had a special bond with Nagisa, she might become withdrawn and filled with rage, vowing to take revenge on whoever was responsible.
I don't think so. There haven't been any official announcements or indications that the HxH manga will continue. But you never know, maybe there'll be a surprise in the future.