Is Kalanthes believed to be an acheronian? I wouldn't think so. He is said to be "the last acheronian style priest" but that's merely circumstantial since he is the high priest of the cult of Ibis which originated in Acheron. I would think that the surviving acheronians have little to do with the Ibis cult, more because the cult itself would be careful not to keep such a relation, being under the watchful eye of the cult of Mithra, after all, they defected Acheron to survive, and I think they would do it again.
I also think Set would have had a much powerful cult in Acheron. The hint that acheronians held 9 mysteries, indeed indicates that their devotion to the Old Serpent was much greater than the Stigyans'. Te loss of the nysteries might also be because that knowledge was lost to history, considering the serpent people held 12, but Acheron in general is considered to be much greater in sorcerous things than any other recent nation. I would think remaining acheronians would hold the cult of Ibis in great contempt for their past treachery, and the cult might still keep something that the acheronians believe to rightfully belong to them.
Also, although the original cult of Ibis that would betray Acheron, was comprised of acheronians, in time this race would have become recessive, as they mixed with the nemedian. I think their survival would have depended on it. Ibites first loyalty is to their god and their studies. Ibites are scholars first and foremost.
There is one thing that intrigues me about the cult as fleshed out in Faith and Fervour, and this is their higher mysteries:
The fifth Mystery of Ibis is that Ibis is a demi-urge and is not the true light of the universe. The Keeper of the Fifth Mystery may learn the Summonings Sorcery Style (Master Words and Signs, not Demonic Pact) if
approved by the High Priest of Ibis – the character must swear an oath not to cast curses in public upon pain of torture and/or death. The Keeper learns of the Source of All Things, the Light of the Universe and gains a +2 bonus on saves versus Corruption.
The seventh Mystery of Ibis is that of immortality. The sacred geometry of the soul is discovered, as well as the horrible secrets of bodily regeneration and/or rebirth with an intact memory/soul. The consciousness of the Keeper of the Seventh Mystery of Ibis is expanded and the Keeper may behold the Immortals. Further, the full truth about the hieroglyphics are unfolded – they are not just for writing – they are magical keys to Immortality. The cause of death is the love of the body, of the physical. Immortality comes from knowledge of the true self. The Keeper may learn the Immortality style of magic, as a bonus Sorcery Style.
So, at one point, they learn of the True Light of the Universe, and that knowledge empowers them against corruption, when they learn summonings they can only learn the Master words and signs first, so they are forbidden to undertake demonic pacts, but later on, they learn the secrets of immortality, which invariably require human sacrifice, as per that sorcery style. So there is this dichotomy in the practices of the cult, but curiously, rather than strike me as contradictory, I think it suits it very well somehow. It may be because of their philosophy:
The Cult of Ibis teaches that men are little gods, that the spirit within transcends the flesh. The cult teaches that man is born in order to commune with nature but that the spirit is still god-like – but that men have forgotten how to touch their godlike natures. Alchemy and magic are part of a god’s power and knowledge is the ultimate power. With knowledge comes godhood. If he partakes of enough knowledge, a scholar becomes as the gods are.
Again, the sixth mystery may hold the key to their true teachings. A glimpse to all the knowledge of the universe may be world view changing.