If it is the moment that Luffy is teleported at the exact moment that his past self arrives in MF, many things would happen – Luffy's disproportionate strength would take the navy by surprise, and could take a while until they give him the attention he deserves; Kizaru would probably be the first admiral to confront Luffy, especially in a scenario where he would directly activate Gear Fourth and fly towards the execution platform at super speed. Luffy, knowing Kizaru's strength and Kizaru, not knowing Luffy's, would make Luffy quickly stun him (Maybe with a one-shot by a Over Kong Gun). After Luffy defeats Kizaru, everyone would be surprised by his absudacious strength, maybe even WB didn't trust him much (Which could be converted later by Luffy's charisma and his friendly situation with WB's crew, and Ace). Certainly the focus of the big shots on the navy side (Mihawk, Doffy, Aokiji, Moriah, Sengoku, Garp, etc) would not be limited to commanders or attacking randoms (as in canonical history), but focusing on stopping Luffy. However, Luffy would be focused on saving Ace rather than fighting, so he would likely use Gear 5's versatility to defeat his enemies as quickly as possible (or outwit them) in attempt to save his brother.
I believe that in the end, Luffy would obviously save his brother, and with the WB pirates in the war, they would defeat the entire navy, and Ace/WB would never have died. After all this, Blackbeard would appear and also be quickly stomped.