. The only difficulty with this scenario is convincing the rest of the Franks to reinstate the Merovingians in the event of a Carolingian collapse, rather than one of their own. That, I believe, is where the problem lies.
A collapse of the dynasty in the early 770s isn't too unrealistic. The only living members of the dynasty that were in some way descended from Martel in 771 were:
- Possibly Drogo, the first Karloman's son. He was stuck in a monastery and could have died sometime 754-770s.
- Adalhard and Wala, Charlemagne/Karloman's uncles through Pippin III's half-brother Bernard. These were both abbots by the 800s, I don't know how old they were in 770.
- Tasillo and his son Theodo, duke and heir to Bavaria. (Through Martel's daughter Hiltrude, so not sure if they count?)
- Charlemagne, plus his sons and bastard daughter(s?).
- Karloman, plus his two sons and daughter.
- Gisela, Charlemagne's sister. Not valid for succession.
This isn't especially many and it is conceivable that a number of them could die or be too young, which would throw the entire dynasty's position into chaos.
Because while Carloman was a friend and ally to Desiderius, it was always an alliance of convinience. And when Desiderius inevitably dies of old age, any direct relationship the Franks had with the Lombards will dry up. That doesn't mean Adelchis would be hostile or cold towards Carloman; indeed, he may well favor him for supporting his father against Charlemagne. But just how their relationship would play out with both in the respective seats of power of Europe would be anyone's guess.
Of course, if the Merovingians were to cause a de-facto civil war in pursuit of the throne, then the Lombards may well throw their support behind the Carolingian loyalists, leading to any neo-Merovingian Francia politically opposed to the Lombards, bringing us back full circle.
The fact that Gerberga (Karloman's wife) and children fled to the Lombard court instead of Bavaria, which would be a closer relation, suggests that the alliance was at least a bit more than 'convenience'.
If the Merovingians decide to fight Charlemagne only, the Lombards would be most likely willing to fight with Theuderic. If the rising is against Karloman, the Lombards will defend Karloman's position. It is hard to know what would happen if both brothers were attacked.
Then there's also the fact that no one knows what happened to Childeric's son Theuderic. Supposedly he died in the 750s, but there's no confirmation one way or the other.
Childeric is known to have died in 755, give or take a little. Theuderic would have been in his twenties by 771. He survived infancy (else there would be no record), so chances are he would survive until middle age. In OTL Charlemagne just imposed himself so much that everyone forgot there actually was a son of Childeric still hanging around somewhere.
I think the best bet would be to have the nobles rebel against Pepin the Short's attempt to crown himself King in 752 and force him to back down or remove him entirely. In that case the Merovengians could at least continue as titular rulers. Or discredit the Carolingians enough to have placing Prince Theuderic on the throne become a good idea, like say another Muslim invasion ending in Carolingian defeat or a loss to the Saxons. Something that makes it seem like they've lost divine backing.
The Pope was the one that gave the final vote on whether Pippin could become King or not. Knowing Papal-Lombard relations, it is possible that Pippin's plan would be blocked if the Lombards threaten war. The other best event to use is the Jan. 772 sack of Deventer by the Saxons, in line with a Karloman-lives TL.
Sure, after Pippin's death there was a conflict between Charlemagne and his brother Carloman but I think this means the local nobilities would have more likely favored one brother over the other. I'm also not sure any of the nobles would be interested in bringing back the Merovingians on the throne.
If Karloman joined his brother in 768/9 to fight the Aquitainians, Lupo and Hunald (assuming Hunald doesn't get captured) would want to support neither of them. A Merovingian would look appealing in their eyes.
(Apologies for long post)
- BNC