REVISED TO BE MORE CLEAR ON MAR 19:
You also write, “There is more pain (more of these chemical reactions based experiences) in the 5 headaches than there is in the 1 whether or not they occur in a single subject. I don’t see any reason to treat this differently then the underlying chemical reactions.”
Well, to me the reason is obvious: when we say that 5 minor pains in one person is greater than (i.e. worse than) a major pain in one person” we are using “greater than” in an EXPERIENTIAL sense. On the other hand, when we say that 10 neural impulses in one person is greater than 5 neural impulses in one person, we are using “greater than” in a QUANTITATIVE/NUMERICAL sense. These two comparisons are very different in their nature. The former is about the relative STRENGTH of the pains, the latter is about the relative QUANTITIES of neural impulses.
So just because 10 neural impulses is greater than 5 neural impulses in the numerical sense, whether the 10 impulses take place in 1 brain or 5 brains, that does NOT mean that 5 minor pains is greater than 1 major headache in the experiential sense, whether the 5 minor pains are realized in 1 brain or 5 brains.
This relates back to why I said it can be very misleading to represent pain comparisons in numerals like 5*2>5. Such representations do not distinguish between the two senses described above, and thus can easily lead one to conflate them.