I haven't watched the marineford but I'm pretty spoiled already so I don't really mind being completely spoiled at this point, But there still some points I might not know so make sure to tell me.

Eden Nathaniel Zero my OC, Would first be confused about what world he has landed (Eden has had multiversal adventures prior to this so he immediately knows he's in a different universe just from his artstyles difference) Seeing and learning that there is going to be a war Eden would immediately try to seek knowledge about war and what is going on in the world he would probably encounter and find someone like coby to ask for help. (since Eden has gone on many multiversal adventures he can recognise which universe is directly connected to his own and which is a unique world Knows this universe is different from his own so he can mess around here without causing a rift in reality)

Eden would probably try to stay away from the war As he doesn't knows if this ace person is a good guy or if the marines are trustable but at the same time he wouldn't want to just let people die over the life of a single person, Eden stands over a building watching people die pointlessly, Beaten up soldiers getting trampled by each other, He wants to stop the war and he will do anything to do it.

During the war, Coby Finds Nathan again on the building as he asks who he is and what is he doing here? Coby asks if he's a pirate to which Eden responds with a simple no, Coby asks why is here and what does he wants. Eden replies he isn't here by a choice and he just wants this war to end, Coby looks over the battlefield watching people die makes him shiver a little.

Eden looks over at Coby and asks which side is the right side? Coby is confused and after short while he replies, "I don't know, I'm a Marine and I know I should say that we are the correct side, But right now even I don't know if I'm correct." he then adds "to be honest I don't think there is correct side, both sides could be wrong and maybe neither sides are wrong but i don't know."

Zero then looks at him and asks one final question "Do you think Ace should die?" Coby is silent he looks at the horizon of death and bloodshed and nutters a simple and small "No" A smile appears on Eden's face as he replies "alright then" as he jumps off the building and rushes across the battlefield and straight to the platform where Ace is and breaks his chain as he declares "he's going to end this war."

Rest of it is simple using speed manipulation Eden helps the white beard pirates escape along with ace, due to Eden speed being far superior to anyone in marineford currently Eden is able to make through the battle completely unscathed, the only opponent Eden has to worry about is Kizaru but Eden is no stranger to outspeeding light itself so I think he has slight bit off edge here. Plus Eden can hold most marine all by himself too purely by his speed manipulation and stamina so escaping for others just becomes easier.

Edward newgate aka whitebeard and Ace survive and blackbeard escapes like usual but aokiji still decides to join him everything else goes like usual , Ace and luffy part ways again as ace and whirebears decide to stick together and remain hidden for now as marines would be after them while continuing to look for blackbeard, and Jinbei and luffy return to Amazon lily, as luffy decides to train for now just how it goes in canon.

Luffy will ask for Eden to join but Eden immediately refuses saying he needs to find a way to go home. So Eden Just travels across the seas looking for any mysterious portal or rift to find a way to return home.