One aspect that could make the Elizabeth Afton novel interesting is her character. If it's like the rest of the Five Nights at Freddy's related works, her story might be full of twists and turns. Her connection to the animatronics alone can be a fascinating point to explore.
I'm not very familiar with a specific 'Elizabeth Afton novel'. But generally, if it's related to the Five Nights at Freddy's universe where Elizabeth Afton is from, it might be filled with horror and mystery elements, perhaps exploring her character, her experiences in the creepy animatronic - filled world, and how she fits into the overall dark story of the franchise.
Gon would probably be angry at first. He has a strong sense of justice and being kidnapped against his will would surely make him mad. He'd try to fight back immediately if he had the chance.
In some fanfictions, Gon might initially be shocked. He has seen Hisoka as a somewhat eccentric but strong ally. When he discovers Hisoka's more sinister side, like his love for fighting to the death and his disregard for normal moral boundaries in battle, Gon could be conflicted. He might feel a sense of betrayal because he thought Hisoka was more on the side of 'good' in their adventures. However, Gon's own sense of justice and his unwavering belief in his friends could lead him to confront Hisoka about his true nature.
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.
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.