I want to throw this out here for discussion- how do you see things being different with the following changes to the "start conditions" of the order:
Diedne warns Charlemagne of the Pope's plotting with his brother, as such Charlemagne remains married to the king of Lombardy's daughter and does not defend Rome from an invasion by Lombardy.
upset with the church over these issues he does not make convert or die an aspect of his war with the Saxons, allowing them to keep their faith
Shortly after Tremere is thwarted in taking over the order it is revealed that it was Diedne who undermined the plot.
Title was chose to separate it from other potential alternate timeline discussions in the future.
I suspect it leads to a considerably less Christian Order of Hermes, probably to its own minor detriment. Sheer survival instinct will keep it from taking on the Church directly, but a lot of local friction between pagan covenfolk and the church, and between Christian covenfolk (supported by the local church) and the magi, and adding ongoing religious tensions between pagan and Christian magi (the latter probably still a majority,, but a much smaller one viewed with constant suspicion by the rest) to the internal politics of the Order...
It sounds like a recipe for an Order with constant small strife wearing at it and its members and resources.
I have a really hard time believing that would be enough to stop Charlemagne from changing his convert or die policy. This is the man responsible for cutting down the World Tree in Northern Germany and igniting the viking attacks for the next 250 years.
That said, I still see the order being largely Christian and it not changing much.
As for Diedne being responsible for the sundering - hmm. Hard to say. My understanding is that they were generally quite aloof and didn't interact with the rest of the order much. Depending on how paranoid the other Houses were, it could mean anything from "No Change" to "Let's try to be nicer to the Diedne" to "Watch out, who knows what they'll do next?"
Keep in mind this is likely to change the whole of mythic europe, not just the order. Without Charlemagne committed to a join or die campaign but instead just avenging an intrusion into his Frankish kingdom, plus the fall of Rome to Lombardy, right as the schism between Catholics and the Eastern orthadox was beginning could lead to further schism in the Christian faith, with a Roman Catholic church under Lombardy's thumb, an Avignon style papacy being founded somewhere else (likely not Avignon), and might even reverse the assimilation of Celtic Christian elements into Cathlicism to lead to it becoming a fourth faction of Christianity.
As to Charlemagne, religious tolerance had been the norm for centuries and there were certain pagan practices which Charlemagne himself participated in (especially regarding his first maybe-wife who was joined with him under pre-christian ceremonies). Realizing he had been manipulated and (Diedne's version) betrayed by the Pope could significantly change his approach. In theory there could even be some form of mind control by Diedne, with the specific intent of reducing the spread of christianity.
As I see it she is known to be advising Charlemagne and suspected to be manipulating him with magic.
Keep in mind that Francia is still expanding under Charlemagne's rule, and not knowing the alternate history there is no need to save it.