The "Ancient Baatorians" could be done as born from the blood of the Serpent when it first appeared in Baator-
when the Fall took place and Triel/Baalzebul, Eblis, and Belial, along with a huge host of celestials (secretly manipulated by Lucifer) arrived, they drove most of the minor "ancient baatorians" into hiding. Big-name ones, survived though- Leviathan, Lilith, Mephistopheles, Dispater, Mammon.
The "ancient baatorians" of WOTC Nine Hells, would be represented as Abominations/Elder Evils in the Dicefreaks version. "Ordinary" weakest abominations for most- but a few manifest in the world as Elder Evils.
You could even keep the story from Elder Evils, of one claiming to be their ruler (Zargon), being cast out by Lucifer/Asmodeus, but always regenerating from his horn.
As for Baernoloths- you could do what 4E did, and make them fairly ordinary- only claiming to be a progenitor Evil race.