I think my problem is this: “1. So when two life streams experience rebirth at the same time, with 100% of the same impurities, kamma energy, every rupa and experience from the past, etc., everything is completely the same.”
We are not (at least I was not) talking about TWO lifestreams.
- It is the same lifestream switching from one existence (bhava) to another. For example, we will not be reborn human forever. A human bhava may last thousands to even a million years.
- I addressed what happens when that human is reborn as a Brahma at the end of a human bhava. Within that human existence, a SINGLE gandhabba (manomaya kaya) can be reborn many times with different physical (human) bodies. But at the end of that human bhava, that gandhabba dies, and a Brahma is born simultaneously in a Brahma realm. Since a Brahma does not have a physical body, it will live its entire existence with that manomaya kaya.
Let us get this sorted out first. If this is not what you had in mind, please explain what you were thinking about.