Let's assume the story up until this point hadn't changed. Ahsoka still left the Order. Obi Wan sneaks aboard Padme's ship to go to Mustafar. The only significant difference is that Ahsoka either sneaks aboard Padme's ship with Obi Wan, as a plea from Obi Wan to help him save Anakin, or she meets him on Mustafar, again after speaking to Obi Wan.

Vader would still force choke Padme and knock her unconscious. Obi Wan and Ahsoka would duel with Vader together, both begging him to come back from the dark, to repent for his actions. Vader would have nothing of this. Ahsoka would be on the attack against Vader, while Obi Wan would act as the shield, defending both himself and Ahsoka from Vader's lightsaber blows.

At some point, Vader will feel overwhelmed. He's duelling his master, the Jedi that trained him and therefore knows how to defend himself against Vader's blows. He's also battling his Padawan, the student that he himself taught. The only way to even the battleground is by separating his teacher and his student. When the time is right, Vader force pushes Ahsoka off a ledge to a platform below. By that time, the station on which they are duelling starts to sink into the lava. The duel continues as seen in ROTS, with Obi Wan eventually winning, and with Ahsoka realizing she can only flee. She runs back to Padme's ship to tend to the senator. Obi Wan reunites with them and they flee the planet before Darth Sidious arrives.