The Kaf Hachaim (55:31) quoted by the OP brings many opinions who hold that all participants in the minyan may say Kaddish.
Would two people include the one saying Kaddish or not?
No. Clearly he holds that these two people may not be saying Kaddish.
In a minyan where everyone is saying Kaddish, why do we need anyone to "answer" at all, based on the meaning of the word imru which means "they should say" not "they should respond" (to someone else saying). Thus, the minyan has more than 2 (as a matter of fact, it has at least 10) people saying "Amen", already!
Indeed, many opinions hold that the same people saying Kaddish count towards the ten people required to answer "Amen", and they hold precisely that, as explained in the Kaf Hachaim, that they should have in mind two things when saying "v'imru Amen":
- That they're telling the congregation to say "Amen".
- That they're answering "Amen" to what the congregation is saying "v'imru Amen".
However, despite this, the Ben Ish Chai1, and based on this the Kaf Hachaim as well, recommend that at least one person (or two people, according to the Kaf Hachaim) should be available to answer. The reason seems to be (based on my reading of Rav Poalim), that it would appear to be a joke if they all say "v'imru Amen" and no one says "Amen"!
1 The Ben Ish Chai references to what he writes in his sefer "Rav Poalim", however upon looking in Rav Poalim I could not find where he says that at least one person should answer.