Yeah, well..
My initial point and i stand by it an Apprentice will be on the ship that's an undeniable fact.. And Bonney is
the only one that make sense on that list.. I can't think of anyone else that can actually fend for themself..
Kuma is a minor issue that can quickly be resolved..
Example.. Kuma is actually slowly dying since he got part of his head ripped off by Akainu, he could live for maybe another month, half a year, a year, but restoring his Soul and Mind will in reality speed up the process of his death.. It's the same as Oden boiling in oil for one Hour, even if he survived the whole time and Kaido didn't kill him, He was passed the point of no return.. Making Kuma a mindless and soulless machine is uncharted territory after all, it was a experiment..
If i can come up with this, Oda can easily think of 10 different alternatives to free Bonney and join the Strawhats..
Edit: Oh, i didn't see the cool analogy with Whitebeard, i doubt Whitebeard would have survived with that same injury that they both got from Akainu?..