Yep. Since I first saw the alphabet theory and saw people arguing that it meant we're still getting 2 more, I always thought it was odd that no one seemed to consider that Going Merry and Vivi might already count towards this. But I've never liked any of the 13 Straw Hat theories outside of the Ichimi one, so I'm pretty biased against it.
Agreed. Oda could have him declare he's joining at any point and I wouldn't blink. Law was even there for Ace's death and helping Luffy at his lowest point, which I think is pretty key at this point in the story. Oda just needs to flip the switch from "not Straw Hat" to "Straw Hat." But I think Law will most likely fall into the major-ally-not-quite-crew-member group with Kid and Hancock.
Jinbe did have steering the ship from Impel Down being the earliest indication of him being a helmsman. I would also argue that his unwillingness to fight Whitebeard was also the first hint at what his dream would be. I don't remember if he was explicit about why he wasn't fighting, but the foundation was at least laid in Impel Down. Hancock, meanwhile, hasn't shown any real inclination for a position outside of being a captain. Yamato is similar, but it's still relatively early for Yamato and she hasn't had a chance yet to sit down and show a non-combat skill she can contribute. I've also always argued that being an apprentice would be fitting for Yamato given her father's past and Inuarashi calling Oden essentially an apprentice to Whitebeard, her intrigue in the stories of other pirates, and she's still inexperienced. For Hancock's dream, the only thing outside of Luffy is most likely taking down the Celestial Dragons, but that would align her more with the Revolutionaries than Luffy. There's nothing wrong with that being her dream, it's certainly more concrete than Usopp or Jinbe's, but she would need more development in that direction first. Hancock I would say is just below Yamato as far as prospective crew member goes. Both need more work, but Yamato has more going for her than Hancock right now. In Hancock's favor is that she's pretty much guaranteed to be very relevant in the near future, so she'll have to be addressed at some point.