They would probably reunite, but most certainly in the original intended time (3 days). They obviously wouldn't be strong enough enough to make it in the New World, so they would need a bit of luck to strive there. Plus, there could be probably a split between them, as Sanji would never accept Zoro as a captain and vice-versa.
- Zoro would probably try to keep up with his dream, with the rest of the strawhats or alone, but he would probably fail sooner or later (he isn't good with directions and his pre-skip version would've trouble against many of the New World's dangers). Considering Zoro's personality and drive, he would 100% meet death;
- Nami would either be under Zoro or Sanji, or would end up staying in Weatheria in the best-case scenario for her. If she continues pursuing her dream, she would probably end up dead IMO (even with Zoro and Sanji alongside her) or, at the worst-case scenario, enslaved like she was by the Arlong Pirates;
- Usopp IMO is the one that would probably go back to his homeland (Syrup Village), as he intended to do after fighting Luffy in Water 7. Luffy was his inspiration to go to the sea, so without him I doubt that he would continue. If he continued with his adventure, he would probably end up dead like the others;
- Sanji would continue his adventure or would return to the Baratie, but would eventually be captured by Germa 66 (or a Big Mom's subordinate) and be forced to marry Pudding, just as happened in the story (however, he would 100% end up dead in this alternate timeline);
- Chopper is an unknown for me. He seems to be closer to Robin, so he might stick up with her. As a result, he has a high chance of ending up dead too;
- Robin is a tough one due to her status, but she has one good scenario for her: staying with the revos. If she stays with them, she has a good chance of surviving at least until the final battle between the revos and the WG. If she ends up with the rest of the strawhats, she probably dies or get captured by someone (either the WG or a Yonko crew, probably). There's also a slim chance of all the strawhats joining the revos due to the Robin's connection, but I don't think they would align with most of the strawhats' dreams;
- Franky will probably tag along with the rest of the crew (or at least one of them), so he has a pretty high chance of dying early in the New World;
- Brook is probably the same as Franky. He has a slim chance of going backwards after Laboon though;
- Jinbe would never join the strawhats. They probably wouldn't even meet each other in Fishman Island and he would probably have to go solo against Hody and his army, as Fishman Island arc just happens 2 years after Marineford. If he manages to protect Fishman Island on his own, he probably survives to old age.
In the hypothetical scenario where all of them train 2 years like the actual story and return to Sabaody under Zoro's leadership, they would probably die at Dressrosa (or, if they didn't follow the original path, at the first time they face a Yonko crew). None of the strawhats besides Luffy were strong enough to face DoFlamingo at the their Dressrosa versions IMO.
- Return to Sabaody would go like in the manga, no problems here;
- Fishman Island would never happen, they would just pass there untroubled as Hody was still making the preparations to take Fishman Island. Shyarly could possible predict their deaths and dispirit some of them (lol).;
- Punk Hazard could be the end of the line already, it all depends on if they ally with Law or not. I feel that under Zoro they wouldn't form an alliance with Law, so they would need to escape Punk Hazard in order to survive (they wouldn't be able to defeat Law + Caesar + Vergo + Smoker + all of Punk Hazard's forces). Momonosuke and Kin'emon would probably die here;
- If they survived Punk Hazard, Dressrosa would be surely the end. None of them would be able to defeat DoFlamingo, even if they allied with Law. The Grand Fleet also wouldn't be formed.