I think their confrontation is inevitable. Obi-Wan and Yoda must be absolutely convinced that Luke will have a zero chance of succeeding to turn Anakin back to the light side...as well as Vader sharing the exact same sentiment when Luke tells him that he senses the good/conflict in him: "It is too late for me now, son" and "Obi-Wan once thought as you do. I MUST obey my master".
If there was anyone that could get through to Anakin besides Padme and the twins, it'd be Ahsoka. She knew Padme was pregnant. I'm pretty confident that she could infer that Vader=Anakin. I would think that the Rebellion and Ahsoka confronting Vader would be Obi-Wan and Yoda's main plan knowing Anakin's "weakness" to loved ones in danger. Obi-Wan at the end of RotS still feels remorse, blame and sympathy towards Anakin yet ANH Obi-Wan is all whatevs and absolutely convinced without a doubt that Vader cannot be turned back.
I feel as if something has to happen that echoes Luke vs Vader in RotJ, except instead of where Luke succeeds, Ahsoka fails. Emperor force lightning her as Vader looks back and forth between them, same as Luke...this would add more to Luke's success because Vader would have experienced this scenario before and this time thinking "Nope, not this time, Sheev, eff this." Palps knows she's a true-believer unlike untrained and maleable Luke, especially during and after the events of her framing and trial, so there'd be none of that nonsense of Vader suggesting/begging to try and turn her to the dark side to spare her life.
I also think the Ahsoka vs Vader scene is necessary to BEGIN Vader's journey back towards the light side. Fifteen+ years have gone by with Vader thinking he's lost Padme and has gotten stuck in his life-support suit, and therefore has nothing left in the world to care about so he might as well just be the Emperor's puppet, give himself completely over to the dark side and not think about it since it probably helps him dull the pain and loss. Ahsoka's reappearance to him would serve to remind him of who he was, what he accomplished before - she's the personification and culmination of everything Anakin accomplished that is good. Luke wasn't the impetus for these kinds of thoughts - we definitely didn't get that vibe as they're walking down that tunnel in RotJ chitchatting. Re: "I sense the good in you" Clearly, Vader had been having these thoughts for quite some time; he just felt powerless to go up against the Emperor. Ahsoka's return and death would both fuel Vader's reminiscing of the good ol' days so-to-speak as well as absolute fear of what the Emperor would do to Luke if he didn't continue to fall in line.
As for Fulcrum = Ahsoka. The best clue was the episode with those parcels where Fulcrum said she'd mark them with her symbol. The symbol resembled Ahsoka's forehead tattoo/symbol thing.
Another benefit to Ahsoka being Fulcrum and part of the Rebel Alliance: She can serve to solve the problem of why Luke got a Jedi Master to look after him while only public figure Bail Organa got to look after Leia instead of Yoda. Yes mainly, Yoda needed to train on Dagobah, hide his presence in the Force and whatever else, but seems a little too unbalanced and unfair that there's no qualified bodyguard/protector for one child but there is for the other. It'd be pretty clean for Obi-Wan and Yoda (or just Yoda) to seek out Ahsoka, explain what happened and for her to without hesitation volunteer to look over Leia from a distance. Remember, she's already aware that Padme was pregnant when she left the Jedi Order. When everything went down in RotS, she would naturally figure out better than most that Vader=Anakin, be motivated to be proactive (Rebellion, save him, whatever else) and put two and two together and wonder: "What about Anakin's child?"