The guild would likely be ecstatic. Gray would probably make some snarky comment about Natsu finally growing up, but deep down he'd be happy for his friend. Erza would be all for it and might start planning the wedding immediately, giving orders to everyone in the guild to help out.
Master Makarov would give his blessing with a big smile on his face. He'd see it as a sign of the guild's growth and family - like nature. The guild as a whole would probably throw a huge celebration, with lots of food, drinks, and merriment. And they'd all look forward to the wedding, which would be a grand event in the Fairy Tail world.
Some of the more mischievous guild members like Gray might tease them a lot. He'd probably make jokes about it every chance he gets, much to Natsu and Lucy's annoyance. Happy, on the other hand, might be really confused at first but then start to see it as a sign of their closeness and be really happy for them.
The guild as a whole would likely mobilize. Erza would start formulating a strategic plan. She'd consider the strengths and weaknesses of Acnologia and figure out the best way to approach the situation. Gray would also be ready to fight, using his ice - make magic to support the rescue mission. They would all be united in their determination to get Lucy back safely.
She would fit in very well. The Fairy Tail guild is all about family and acceptance. Given her parents are Lucy and Natsu, she'd be welcomed with open arms. She could learn from the older members like Makarov and Mirajane, and bond with the younger ones like Romeo. Her magic skills would also contribute to the guild's strength during battles or when helping the town.
The guild would likely be shocked at first. They consider Natsu as one of their own, so his attack would be completely unexpected. Some members might try to talk him down, like Lucy. She has a close bond with Natsu and would probably try to reason with him.
Lucy might be really surprised at first. But then she'd probably blush and say yes. After all, she and Natsu have a strong bond already.
One possible plot twist could be that Lucy initially refuses Natsu's proposal because she fears it will change their dynamic in the guild. She might think that getting married will make them less focused on their adventures and more on domestic life.
The guild members' reactions would vary greatly. Happy, being Natsu's closest companion, would likely be excited about the fanfiction. He might fly around the guild hall, telling everyone about it and adding his own little comments about how accurate or inaccurate it is. He would definitely be protective of Natsu's image in the fanfiction, especially if it's not a positive portrayal.
At first, they might be in disbelief. The members of Fairy Tail see Natsu as a core part of the guild, so it would be hard for them to accept that he has betrayed them. They would probably try to talk to him and find out what's going on. Some members might be angry immediately, thinking that he has turned his back on their family. But others, like Lucy, might be more hesitant to believe it and would look for a way to resolve the situation without conflict.
The Fairy Tail guild would probably be very curious about Naruto at first. They might be amazed by his strange abilities like running up walls and creating clones. Some members, like Lucy, would likely be a bit cautious, but also intrigued. Natsu would probably be eager to test his strength against Naruto right away.
Their wedding would be a grand affair. All the Fairy Tail members would be there. Natsu would be in his best suit, looking a bit uncomfortable but happy. Lucy would be in a beautiful white dress, with her Celestial Spirits perhaps helping with the decorations. There would be lots of food, music, and dancing. Gray might accidentally freeze something during the celebration, and Erza would keep everyone in line. It would be a day filled with laughter and joy.