Bones and Bone Marrow

[Haipule]

If I say that the rib bone of a human contains multipotent stem cells: science doesn't disagree.

Correct. Sort of.
The biggest detail, you neglected to mention, is that the quantity in marrow is very small. Plus, those stem cells are not "potent". They have a limited range as to what they can become.

If I say that that bone will grow back if the surgeon leaves the sheath that houses it intact: science doesn't disagree.

Bones are 'just bones', they don't come inside a sheath. If you mean the marrow inside the bone will regenerate, then yes, that is correct.
If you're saying that a bone will grow back if it's removed, science totally disagrees. A removed bone is gone, permanently. And a removed rib will not "regrow" into a human, it will just die. At even a rediculous best case scenerio, the rib will just grow more rib.

Rib bones do grow back if removed. It takes 2-3 months and will reattach to the spine and be as good as new once healed. If a persons face is destroyed in an accident, the surgeon will remove a rib to reconstruct the face. If they need more bone, in a few months they will remove the same rib which grows back. They will remove it as many times as is necessary. 
Google it.