Of course there also other possibilities. The mentioned Uncle of Kukaku's and Ganju's that would be unhappy with sending Ichigo to the Royal Palace doesn't necessary have to be a member, or bear the name of the Shiba clan. It might not even be a relative of the Shiba's but perhaps an uncle-like-figure.
Still, for that "Uncle" to be displeased with sending Ichigo to the Royal Palace, it certainly limits the possibilities.
BTW. I was rereading some chapters and I think that I realized how the Zansmith rebuilds Zanpakutou. The thing with Asauchi attacking Ichigo and Renji wasn't a test, but a procedure in order to rebuilt it. You can notice that after Renji completed his training, all that was lying around him were Black Asauchi:
Yes, they turned from the Clean White Ones, into Black Specialized Ones. Basically the procedure to create your own Zanpakutou is by training and pouring your power into a clean Asauchi, this is exactly what this training was about. Thanks to completely turning them black, which means turning them into your own Zanpakutou, the Zansmith have the material, type and power within them, the only thing remaining is to remodel them all into one Sword (that's probably why the new sword is better) using the supreme reijutsu that they have within the palace.
The reason why Ichigo failed, is likely because after 3 days and night, not even one started turning black, not even one aided/deemed him worthy. That's why it was pointless to continue the training the way he were.
It's also worth noting that Urahara's special doll for Bankai training works only for 3 days and quite likely works very similar to what was presented within this training. As it is with all of Urahara's mayor inventions that are based on things created by members of the Zero Division. I wonder then what is the H-cube then, is it something based of SK, a member of the Royal Guard, or a combination of it all ?
It's also worth noting that the next two palace will likely teach about Kidou (The Black Haired Women, those arms-hands might actually be something that allows her to throw multiple spells at the same time) and Hand-to-Hand Combat (The Monk, it was even suggested that he doesn't use a Zanpakutou), since together with Zanpakutou training, they all come together as Shinigami's Power (mentioned once by Aizen somewhere).