Personally, I think the term 'soul' is wrong here (though, depends on definition). As far as I understood those scenes, Spock transferred his knowledge, memories and pretty much what makes up his personality into the brain of McCoy. During the ceremony on Vulcan it was transferred back into the brain of Spock (it must be quite a feat to split two personalities and minds).
The problem was, McCoys brain was far too primitive to handle that kind (and amount) of information. That's why he slowly went nuts and that's most likely also the explanation why Spock had to relearn quite a few things. During the movie we can see that McCoy has problems differentiating between his mind and that of Spock. So I think that McCoy simply would have gone crazy with time, losing his and Spocks mind on the way down the rabbit hole.
Now that I think about it, it seems quite logical that Spock was 'less' human after the ceremony. The priestess needed to sort out what belonged to Spock and what belonged to the human host, not knowing that Spock had ...uuuhhhh... achieved quite some human traits, she simply assumed they belonged to McCoy.