Such a PoD would require either the Visigoths to repel the Franks
Probably : as said above, Alaric II was undergoing some religious "union" at this point before Franks managed to get Gallo-Roman elites on their side and to advance South.
In fact, a good PoD may be having Franks turning Homeans in the early Vth century, instead of turning Niceans in the late Vth. It may neutralize Frankish hegemony in Gaul (while probably not butterflying it in Belgica and Northern Gaul).
The catch is that the Visigoths in Spain were a noble class who ruled over Romano-native peasants and thus there was always a potential fifth column against them.
Quite at the contrary : you never had a native religious revolt in Romano-Barbarians kingdoms (Vandalic religious troubles are more a social struggle than really religious, barely touching the whole population), and the whole idea of a native religious threat never was really taken in account.
If something, the threat was more about other ruling classes, regardless of their origin (as Hermengild in Visigothic Spain) than peasants (critically with the usual commitment of Romano-Barbarians kings to preserve, as heirs of imperium in Western Romania, a certain form of "religious plurality")
The whole idea of an ethnic ruling class was prooved wrong too many times to be maintained : Barbarians were a mix of different groups, and Roman elites weren't the last to get integrated partially (Syagrii in southern Gaul, Vicentus in the Ebre, etc.)
In addtion, it was the convention of the era that wives adopted their husband's version of Christianity.
Less one's husband than his
sippe's, as can be pointed by the choice of support in religious establishment by Brunhilde. Basically, it's about more joining a factional/clanic group "values" (the aformentioned identitarian marker) than individual switch.
And giving that the percieved differences between Homeism and Niceanism were...say limited, usually a real conversion wasn't necessary (with exceptions as Brunhilda, but that's maybe more a litterary device from Gregorius to paint her as a great queen*, as nothing his aid about Galswintha). It's more a change of practices, made easier by the vagueness of Homean "dogmas"
*After all, Gregorius doesn't stop to paint "Arianism" as diabolical, which is to be expected from a bishop, but doesn't really paint the political/social reality in its entierty
The Roman Catholics had a track record from breaking this from a Frankish princess whose name I can not recall who married a Visigoth
I suppose it's Ingund?
That her union with Hermeneglid may have played a role is possible (but remember she was particularly young at this time, and not really able to play a decisive one), but eventually Roman pressure (Hermenegild and Gondovald's revolts participating to a same strategy) and critically Hispano-Roman pressure (Reccared didn't have to marry a Nicean woman to switch, after all) probably played more, while Gregorius is, unsurprisingly, mostly silent on it.
to St Margaret of Scotland.
It's a different case there : at this point, Roman church was definitely a thing, and we're more in the final decline of "national" western churches than a real dogmatic change (these being essentially pastoral at this point, the differences between Scottish and Roman practices being really limited).
A union between the Homeism and Niceanism would only work if the Roman Catholics do not "cheat".
Thing is, you didn't have Romans Catholics to speak off up to the VIIIth century. Papacy was essentially a moral power, without real pastoral or dogmatic (and of course, not political) power in Romano-Barbarians kingdoms were the chiefs of the "national" churches were the bishops and, of course, the kings; as pointed out by the many regional synods and councils.
Meaning that, with a Visigothic hegemony (and critically keeping dynastical succession in Gothia), the center of decision would be in courts, not in Rome (that would be too busy being kept under Ostrogothic control to really have its say).
I admit that a form of union between Homean and Nicean isn't an obvious outcome (as said above, it's only the "best" I could find without Late Roman Empire holding by Homeism), but it's a possible one and already engaged IOTL by Alaric II : it would certainly have been at least partially successful, eventually, giving the lack of "radicality" in Homeism. Of course such union, because of that, would have been made in favour of a more well defined Niceanism.
