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The pronoun forms of who and 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.

UPDATE:

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.

Whether to use which or in which or out of which or of whom or in whom etc. depends on the main verb. It has nothing to do with BrE versus AmE.

So, the verb incorporate is followed by in or into.

One incorporates some thing into something else.

For example, "He incorporated additional colours into the mix."

I don't think one would say "the spirit is incorporated into individuals". A spirit does not incorporate.

If one is describing individuals and the idea of spirit, one might say:

individuals who have accepted the spirit [in a spiritual sense].

An individual into whom a spirit has entered. [psychic powers lingo]

[Edit] The original text uses a very specific meaning of "incorporate", as in this excerpt:

Before Dr. Fritz started to incorporate into Kleber Aran he incorporated in a few select individuals. The first and also most popular was José Arigo (José Pedro de Freitas).

In this case the literal "spirit" (ghost) of Dr. Fritz has "entered into" the body of some person, and "incorporate" is perfectly acceptable term. Note that "embody" can also work, (although it tends to mean something else when used in this kind of context).

Note this is often called "channeling" as in some person channels some spirit .. but again that's slightly different from what is implied by "incorporate". However, this is all jargon used by psychics, mediums, and other mystical practitioners. Jargon does not necessarily have the same meaning among everyone in the field, and (without context) may sound strange to anyone unfamiliar with the field.

The use of who vs. which is a matter of style, and possibly dialect. There is no right or wrong, so it's kind of a pointless argument.

Certainly some dialects might prefer one over the other, but language changes over time, and even the most formal of modern dialects would probably be considered intolerably rude by formal English speakers of a century ago. Arguments over what is proper all depend on who you're speaking to.

However -- in this particular case we're talking about the embodiment of a spirit into a human being, and I personally think "who" is the best option:

There are only a handful of individuals into whom this spirit has chosen to incorporate.