Luffy uses Gear 5 in a significant battle towards the later part of the manga. But the exact chapter or arc can vary depending on how you're reading it.
It would likely make the storyline more exciting. Gear 4 Luffy is extremely powerful, so his presence in Marineford would change the power dynamics. He could defeat more enemies quickly.
I think it's in a relatively recent chapter, but the exact one might vary depending on where you're reading the manga. You might need to check the latest updates.
Well, in the fanfiction, Gear 4 Luffy might have had some new and unexpected interactions. Maybe he could have teamed up with some other pirates in a different way. Since he was in such a powerful form, he could have inspired more of the pirates to fight back more bravely against the Marines. And his presence in Gear 4 could have also affected the admirals' strategies, making them more cautious or forcing them to use their more powerful techniques earlier.
Luffy's victory over Doflamingo in the manga was a combination of factors. His growth in strength, his unwavering will, and his ability to adapt to Doflamingo's tactics all played a role. Additionally, the support of his friends was crucial in tipping the scales in his favor.