Ace is going to be saved, Whitebeard lives, Akainu will be curbstomped.
It's as simple as that. Yonkou Luffy will tear through the troops with his future sight, easily dispatching of Smoker and some of the warlords like small fries.
The Marines, puzzled at Luffy's unfounded strength, is likely to consider him "as strong as the other legends in this war", and treat him like a big dog.
Mihawk is likely to withdraw the fight with Luffy as he did with Shanks for the same reasons, since Luffy is entirely capable of giving Mihawk actual trouble. If not, they will come to a stalemate match, but since currently, Mihawk doesn't have evidence for FS or Hao Infusion, Luffy is likely to overwhelm him with Gear 2nd. If Mihawk have those, they'd likely stalemated themselves, and Luffy would get past Mihawk with G2's speed to avoid confrontation.
This time, the key isn't necessary for Luffy to free Ace, so he just blast past the bay. He also does not need Ivankov's protection as Kizaru is the first sent to deal with Luffy like before.
This time, however, Kizaru can't seem to land a hit on the strawhat pirate. Surprised, Kizaru isn't quick enough to blitz past Luffy's Haoshoku attack, which severely damages the Admiral with the attack, sending him away. Sengoku is very likely to recognize this advanced form of Haoshoku Haki, and immediately changes his plan to focus entirely on Luffy.
But remember, Whitebeard is there as well. He isn't in the point where he joins the fight directly, but it's likely that he wouldn't use the tides tilt attack so it won't trip Luffy up. Rather, the big bois are going to back Luffy up as per pop's instruction, Marco can hold off an Admiral like Aokiji, Jozu is capable of hold off Mihawk, and Akainu...
Well, this Luffy remembers Akainu killing Ace. So when "Lava Girl" gets on sight, Luffy is going to scream, "AKAINUUU!" like a ************.
Luffy then, is likely jumping, and attacking the platform at this point, but then comes Garp. This time, there is no reason to hold back.
Luffy vs Garp
Garp intercepts Luffy while the latter is trying to jump at Akainu's throat. The Pirate's future sight again allows him to block his grandfather's punch, which is unsurprisingly coated in Armament.
Garp tells Luffy he can't believe Luffy knows how to use such an advance form of Conqueror's so early in his journey, Luffy however, ignores this comment as he begs for his grandpa to let him go, and tells him how he doesn't want to watch his brother die before his eyes again, how he is strong enough this time.
Confused, saddened, Garp struggles, but shakes it off by yelling at Luffy that he will have to defeat him to save Ace. Luffy, like in the original story, uses Gear Second, and prepares to attack with all he's got. Likewise, Garp flinches, as not only the memories of Luffy floods through his mind, but Luffy's desperate words shake him beyond his sense of duty. Garp is struck with a power Jet Pistol, this time, it damages the Vice-Admiral badly as he is sent to the ground. Luffy "wins".
Extending his arms, Luffy finally reaches the three admirals' platform, but alas, only Akainu is there. However, Luffy knows that fighting Akainu before saving Ace is just wasting time, he isn't as naive as before. While the Lava Admiral is preparing to fight, in a sudden attack, Luffy uses his Ryou to destroy where Akainu stood ground, gaining some time to climb the platform and reach the top. His Haoshoku easily dispatches both guards, with only Ace and Sengoku remaining.
Now, Luffy knows Sengoku can turn into a Buddha. He is likely grabbing Ace and jumping the f*ck out of there. Stronger, and with Ryou on the palm of his hand, Luffy uncuffs Ace a little before Akainu reaches them. Now, Whitebeard's shockwaves are keeping the other Admirals at bay, and Ace, with no reason to fight Akainu as of now, grabs Luffy and flies the **** outta there.
The rest of the war goes as the original story. Except Luffy is not that tired. When Akainu talks smack about Whitebeard, Luffy already expects as such, and transforms into Gear 4th: Snakeman. With his immaculate speed, after Ace loses his clash with the Admiral, Akainu is hit right in the face with a Jet Culverin, sending him away.
Luffy yells, "You're not going to hurt Ace again, get him, Python".
Akainu is quickly overwhelmed by the speed, and morfs himself to avoid damage, but Luffy can know predict that, and just changes trajectory to hit the admiral. When the attack is finished, Luffy gets off his G4 state, and grabs Ace.
After some struggle, Luffy and Ace, and Whitebeard leaves.
