The pronoun forms of
which seem to be used more 'randomly' nowadays than Oxford English would allow.
Which of the two is more correct. To my somewhat British ears, which is often used in such cases in colloquial everyday English. However, I would advocate the use of 'who' since refers to the individuals and all individuals are referred to, not only a selection.
THE SENTENCE IN QUESTION:
There are at least a handful of individuals into who(m) / which the spirit incorporates?
NOTE: This sentence refers to a given spirit manifesting/entering into a human's body.
Regarding the definition of 'incorporate into' in the classical EN dictionaries, it would not be correct to say something incorporates into somthing. However, when talking about spirits and mediums, it seems to be accepted terminology. Because when checking through Google Books, there are various authors who have used this expression in this sense.