Sage transformation, while not equal to sage mode, is very similar in the power that it grants. An inability to perfectly balance natural energy, but still able to control it and balance it enough, cause the user to undergo bodily transformations (Jiraiya) which may or may not be useful in combat. Jiraiya displayed an increase in mobility while he was in his transformed state. The point is that senjutsu can cause bodily modifications.
Right on the wikia, it says that Orochimaru developed his curse seals based on Jugo's sage transformation enzyme (I.E, he developed his CS based on Jugo's KG). The source of Jugo's clan power was apparently the dragon mound cave or whatever it is called. What that would mean i that Orochimaru's curse seals are also indirectly related to the snake sages.
It is also mentioned that the seal takes away chakra in exchange for senjutsu; the seal itself likely absorbs the wearer's chakra and adds it to its "pool", gathering natural energy and then supplying the wearer with it. So the natural energy is acquired very similar to sage mode, but is granted to the wearer in a fashion similar (though inferior) to sage transformation. Taking all of this into account, the wearer is basically going through a limited amount of sage transformation.
Sage transformation full and sage mode full are very similar, much like sharingan genjutsu and ordinary genjutsu. A KG aides in the gathering of one, and causes mildly different effects due to the KG. Without the KG, Jugo's clan absorbing natural energy passively would turn them all into statues or films of skin or whatever.
When in sage transformation, Jugo can take the organs and chakra of others and add them to his own body in order to heal himself. Applying this idea to Kimmi, upon entering sage mode, Kimmi attained added resilience to survive removal of his spine and the subsequent healing that would have to take place. His Kaguya-ness is that a spine grew back without him having to snatch it from someone else, but potentially, his ability to survive doing so came from his limited sage transformation state.