@Vongola_Boss_XI:
I understand your points, but I believe they were all addressed during the CP9 saga. Robin did leave the crew behind. She _wa_s wearing a fake or at least forced smile for the first leg of their voyage together on Skypiea. Which all came to a head when the Straw Hats pursued her to Enies Lobby and declared their intention to fight the entire world and the darkness which rules in order to rescue their friend, thus fulfilling Saul's words. The most important thing about Saul's lesson to Robin was to smile through hard times because surely there is a brighter day on the horizon. Even though things look bleak, someday, somewhere on the sea, she'll meet people who won't abandon her. Robin ran away from the Straw Hats because she was afraid they would leave her. But they would never let her die.
Luffy didn't let Robin die after Alabasta and she still betrays him later in Water 7. Despite Alabasta and saving her on Skypiea, too.
The Straw Hats planned to take on the World Government, which Robin inherently dislikes and aligns herself with given the events on Ohara.
I think you're misreading Sauro's words here. I think that you're right in that his intent was to teach Robin to smile through hard times because a brighter day would be on the horizon… but Robin seemingly never saw a brighter day until literally 20 years after that. The majority of her life has been in darkness (hence her morbid predictions). So all of her smiles to that point were to put on the illusion of happiness while feeling despair internally. Everyone she loved was killed brutally right in front of her. She lost everything. There were no happy times to be had.
Don't forget that Robin was fully willing to pin Iceberg's murder on the Straw Hats and abandon them. That's not "for the sake of the crew," but an outright betrayal that they've been gracious enough to forgive.
@Vongola_Boss_XI:
I feel like your reading of Robin's character fits with Spandam's description; with the world's perception of the 'Demon Child'. People always betray Robin because they assume the worst of her. The Straw Hats aren't like those other people and that's really the point. It was never that Robin betrays people. It's that people have betrayed her time and again. Yet, you're still assuming the worst of her based on the horrible image the World Government created of her. The false image that the Straw Hats were warned about by Aokiji, Iceberg, etc. and refused to believe. They saw through the lies and recognized Robin as a true friend.
Robin has betrayed people. She was betrayed at first and thereafter, seemed to feel comfortable betraying those she aligned with (as we see in the last several pages of her flashback).
The initial image created by the World Government was that of a "Demon Child," but at some point she lived to fit the description.
You gotta remember that Ao Kiji knew Robin at 8 — but spoke about her actions of betrayal in the time since he first met her.
@Vongola_Boss_XI:
As for motive, it's really not that complicated. The scholars of Ohara had a part of the puzzle, but not the whole truth of the world. Roger's crew were the only ones in 800 years to discover that truth and it remains hidden on Laugh Tale. Roger told at least part of it to Whitebeard and Toki likely knew some of the crucial details as well. The scholars of Ohara value knowledge, history. You need not look any further than real world historians who believe that knowledge of the past illuminates the path into the future. Robin was hunted by the dark power that rules the world for daring to study the history they want to keep hidden. She doesn't need any secret ulterior motive to want to know the truth. We saw the inciting incident for this desire during her flashback. We hear Clover tell the Five Elders exactly what the scholars of Ohara learned. It isn't everything, but the act of studying said history is already an unforgivable crime.
I think Ohara figured it out. Rayleigh seems to believe so.
Remember, Clover got on a direct line with the Five Elder Stars and they ordered a mass purge as a result of Ohara's knowledge.
The reader is not informed on what Clover and the scholars of Ohara have discovered — nor do we learn what was on the Ohara poneglyph (aside from the name of the Ancient Kingdom and about the Void Century). The only person alive who knows is Robin — and we don't know how she feels about it.
We do know that whatever it was concerns the destruction of the world, as the Marines and World Government have pointed out.
@Vongola_Boss_XI:
In terms of 'The Sixth'; If Oda is indeed saving the chapter title, it doesn't imply any sort of betrayal, unless we also assume Chopper is meant to betray the crew in the future. It could simply imply a significant character moment for Robin in the future where she really proves her resolve or contributes significantly toward Luffy achieving his goal of becoming Pirate King.
Chopper still has time to leave. Oda's treatment of Chopper has been very curious thus far. Forced awakening in Enies Lobby. His new hat might be a treasure map ("X" marks the spot) and is reminiscent of the symbol on the Kozuki Crest/Alabasta Flag/Shandian symbol. Perhaps it is the same as the universal model in Ohara's library.
@Vongola_Boss_XI:
I think Vivi is returning; Have believed that for more than a decade. But Robin is the sixth. Oda has given the Straw Hats numbers in numerous color spreads at this point.
Those numbers aren't always consistent.
Luffy is #1 AND 0. Zoro is #1 AND 2. Usopp is simultaneously #3 and 4. This all comes down to how Oda has chosen to count them.