Luffy only survived that war because WB took care of him, he made paths he protected him and he did everything to get Luffy to Ace.
Luffy was a wildcard that the marines didn't count on and didn't see him as a threat.
If current Luffy would be in the war he would get targeted right away like the other big players. Luffy still wouldn't be able to make a difference since he could barely could go toe to toe with Doflamingo with a lot of help from Law and everybody in Dressrosa.
Current Luffy might do worse in this scenario than past Luffy.
They would have held the marines back til Shanks comes and stops the war. Leaving Ace and Whitebeard still alive.
lol, it was an execution not a war. The pirates made it a war. If Shanks would show up before Ace was executed he would still get executed because otherwise it would defeat the purpose of fighting and it would make the marines look weak.
So if Shanks shows up before the marines finished what they needed to do, he would stop nothing. The marines keep on fighting until at least Ace is dead. Whitebeard might survive.