This is in line with my theory as well. Although I don't think they were warring dragon cults but rather a cult that that was split in 2 because one group may have simply stopped with the dragon worship (due to some unforseen event). However due to the amount of dragon blood drank throughout the generations, their bodies had mutated to horned men/women and those that stayed on maintained their scales.
- If you bring Iron Bull to the final fight with Corypheus or a Qunari Quizzy, he claims that their race was a mistake
- Cassandra talks about the scales when people had too much dragon blood - growing scales and dragon like features
- Quanri supposedly have dragon blood
- The Scaled Ones having a rite of passage that is the same as the Qunari (likely originated due to dragon worship)
- When you ask Iron Bull about the Kossith when he first joins the party, he says that they're of the same race but they (Qunari) had to get away from them and shudders. Maybe he knew something and it was brainwashed out of him? He seemed to have a weird reaction to that topic for a race he knew little about and was not talked about amongst the Qunari
As for the dwarves we really don't know how much if at all interaction they had with the surface. The story hints that they seem to be far seperated so the foreign tongue that they heard may have been ancient Elvish/Tevinter or any other group
*Dark Ritual!Kieran mentions that your blood is not your own as a Qunari, they could be a failed Super Soldier Program similar to the Mabari super dog(made by the Vints), based on Ghilan'nain's LEGO Genetics magic(Halla, Cetus, Giant, I think Griffin was also hinted at(cat+bird anyone?)) or they crossbred with Scaled Ones.
*From Par Vollen Codex:
The pyramids and other ruins of Par Vollen's ancient civilization also feature murals of the natives interacting in a subservient manner with horned beings who are not Qunari. It is believed that the Qunari conquered Par Vollen so easily because the natives associated the Qunari with the horned beings they venerated, and therefore most natives chose to submit to the invaders' rule.
According to the Iron Bull, there are dwarven ruins on Par Vollen of similar age and architecture to Kal Repartha
*Krogan expy? NWN reference? Dragons used to be sapient and they were their Kobold-like servants? Weather got colder and the Fade also changed to reflect that, forcing the Scaled Ones to move underground in order to survive. Maybe also a Taung-Mandalorian relationship with The Kossith?