Asuras became malevolent beings after the Rig Veda, which implies the pre-Gathic period was essentially between Rig and Atharva.
If Zoroaster lived between 2000-1500 BC, it implies the Vedas are significantly older than currently dated if we are to stick him between Rig and Atharva, ie, the pre-Gathic and Gathas sit between Rig and Atharva.
But while Rig considered Asuras and Devas as one and the same, Zoroastrians created the split long before Zoroaster came into the show. Which means Asuras had already become dark Titans before Zoroaster turned Asura Medha into Ahura Mazda. That's pre-Gathic.
Four theories:
1. Vedic system reacted.
Rig - Pre-Gathic (Devas became bad) - Atharva (Asuras became bad) - Gathas (Ahura Mazda).
Split.
2. Vedic system reacted.
Rig - Pre-Gathic (Devas became bad) - Gathas (Ahura Mazda) - Atharva (Asuras became bad).
Split.
3. Zoroastrians reacted.
Rig - Atharva (Asuras became bad) - Pre-Gathic (Devas became bad) - Gathas (Ahura Mazda).
Split.
4. Zoroastrians reacted.
Rig - Atharva (Asuras became bad) - Pre-Gathic (Devas became bad) - Gathas (Ahura Mazda).
Misunderstanding. Brahmins made a translation error, gave the "a" in asura a negative connotation. So suras and devas were fine, but asuras cannot be worshipped. Zoroastrians assumed it was an attack on them and they reacted during the pre-Gathic era.
1, 2, and 3 imply a real split whereas 4 implies a misunderstanding. Zoroaster supported the new meaning.
Of the 3 cases, only 1 (case 2) sees Atharva being younger than the Gathas. And it could be any of these 4 cases.
And 600 BC is when the Gathas were finally written down.
BMAC link is disputed. At best syncretization of cultures. For example, fire was important in the colder climate of Central Asia and the steppes.