As mentioned in Bull Gods of Fronela, I have some pet theories of the origins of the Orlanthi peoples and some of the Orlanthi cults. It starts with this: Guide to Glorantha page 689 - "Later Orlanth and his Storm gods freed many of the Kachisti survivors from bondage."
My theory is that the Durevings were the Dronars of the Kachisti peoples (kinda, see end-note). As circumstantial support: The Book of Heortling Mythology page 148 - "Durev does not go in for fancy stuff, for violence or trade goods or fancy legal talk, or for anything that will send someone away. He works hard for his family, cares for his herd, repairs the houses and makes the household sacrifices." I've been plowing through the electronic copies of my sources looking for a half remembered source linking the peoples of Danmalastan with the wood people (other myths in BoHM link the Durevings with the wood people) - I suspect that what I'm looking for is in paper form only.
Similarly, the Helerings originated as Waertagi. And, possibly, (some of) the Vingkotlings originated as Hsunchen (shapechanging was practiced by the Hidden Kings of the line of Vingkot during the Greater Darkness/Chaos Age).
Back to my theories. Likely there were very few Talars or Zzaburi among the enslaved Kachisti, as they do not make good workers, and possibly no Horali as they are good only for fighting. The Zzaburi became part of the Lankhor Mhy cult (likely mixed with refugee Buseri from Dara Happa and possibly more). The Talars became part of the Issaries cult and maybe also influenced the Dar subcult of Orlanth. This infusion of some basic Malkioni culture and mythology was why the God Learners found it so easy to work with the cults of Lankhor Mhy and Issaries. This does not mean that I believe that the source of these two particular cults is only or purely Malkioni.
End-note: I'm always very careful with origin myths, they tell very important deep truths about people, but they may not be as others around them observe their origins. Another way of saying this is that some older truths may be deliberately forgotten to help preserve the current truth and strengthen people. And, of course, the post-apocalyptic landscape of the Chaos Age meant that many people could not afford to remember much of their origins beyond that necessary for basic survival.