Wouldn't Louis the German win in this scenario? Things seem really tipped in favour of him.
It's not really obvious at this point : without a royal presence to distribute
honores in the west, the WFrancian potentes will likely turn to the next best thing, aka one of Lothar's sons. I agree that they would be unreliable, especially if Louis of Bavaria or his son (for clarity's sake, let's call him Louis of Aquitaine) make promises, but it would make WFrancia more of a battlefield than an asset for anyone in this case, because any kingship depended heavily on a built network with local potentes, something that is yet to build for either side.
In case of a conflict you'd probably end up with a generational alliance between Lothar's son and Peppin II (less because he would give them a huge boon than it would weaken Louis of Aquitaine's position), and maybe later Carloman of Bavaria who was at odds with his father IOTL. In fine, Aquitaine and WFrancia would be at the hands of potentes before anything else.
Especially if Lothar managed to take back territories up to the
Silva Carbonaria (as he seems to have tried, unsuccessfully, to claim during the split), Lothar's sons would be in a relatively good position.
Eventually, I think that any conflict would be quick, at least in a first tie, and ending up with a status quo acknowledging Louis as king of Aquitaine and Lothar II influence on WFrancia, not only because of what I mentioned but because of the "Great Raids" of the mid-850's in Francia by Vikings which really represented IOTL a shock and would imply at the very least a truce between Carolingians.
Is the term referring to a specific aristocracy in West Francia or the whole of Carolongia Europe?
No, it's a broad, generic name as
majores. Majores arguably could be better fit, but I took the havit of using potentes because of Bruno Dumézil and now it stuck.
Wouldn't there be also a Provence/Burgundy?
Not necessarily : as said, it existed from the late IXth essentially as an opportunistic creation by Boso of a non-descript kingdom from his rule as duke of Provence, the general idea being using this non-descript kingship to get Italy. Charles essentially got Provence because there wasn't much to give him.
Depending on the number of sons of not only whoever reunites the Empire but as well the sons of whoever was deprived of thrones (ITTL, maybe the sons of Charles the Bald) that would have a claim to kingship.
The quadriparte ensemble I described is arguably arbitrary but not without cause : it was basically the regions that asked the more for a specific king. Everything in between is dispensable enventually.
So Aquitaine is the only realistic breakaway region? Well what remains is basically the Seine valley + half of the Loire.
Borders aren't really clearly defined at this point : Aquitaine was roughly south of Loire, but its delimitations essentially depended from honores and beneficiarii of the aristocracy on which was tied such and such king.
If I had to give a guesstimate, which I really warn is not a given in any sense, it would be keeping the rough limits of 845, meaning most of Aquitaine including Gothia but without Poitou, Saintonge and Angoumois, but arguably (giving several Gothic potentes had holdings in Burgundy) maybe part of Burgundy as well.
Again, delimitations depend heavily from who supported who, and who promised what among their supporters (local or neighbouring)
By butterflied does it mean it(the coastal area) would end up under Italy?
Or Aquitaine. Or Western Francia. See above point.
BTW, do you have any wild guess at what the macro-political future of this arrangement would be(specifically referring to France and Italy)?
If I had to guess I'd say.
Death of Charles the Bald
- Aquitaine goes to Louis the Young (Louis of Aquitaine)
- WFrancia goes to a son of Lothar, possibly Lothar II, while Lothar takes for himself the region up, not including, Silva Carbonaria.
Death of Lothar
- Louis II inherits Italy and Provence
- Lothar II keeps WFrancia and maybe recovers the part taken by his father (if not more)
- Charles takes whatever remains in Burgundy.