Effects of a united Carolingian Frankish Kingdom

Had Charlemagne and his son Louis not divided the Kingdom of the Franks between Charlemagne's descendents, how could the Carolingian Dynasty, in command of a vast swathe of Western and Central Europe, have progressed?

Would it have collapsed in any case, whether that be due to civil war/rebellions or simple lack of infrastructure? If so, when, and are there any likely divisions aside from what occurred OTL?

Would it have simply resulted in a larger Holy Roman Empire, with French states in addition to German?

Would it have remained as one contiguous feudal kingdom?

How would it be affected linguistically? Obviously, French as it exists today would not exist, but would a seperation into "German" and "French" still occur?

Would it be likely at all to expand? Spreading the Catholic faith to Eastern Europe and/or Al-Andalus via the sword?

How would it have interacted with the Pope, Byzantines, Caliphate, Scandinavians and Slavs?
 
A lot depends on how capable the Emperor is and what divisive factors he has to face are.

But I think its probably going to have trouble holding all of that securely, though probably less so than the OTL-HRE.
 
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