The addition of Catalonia May strengthen Occitan in a direct sense, but it implies a much richer and more powerful France that could implement universal mandatory education earlier. While it may not be the most likely scenario, I think it is absolutely possible that a wave of Frenchification sweeps Occitan away from the rest of France by starting earlier and being more intense than OTL, while Catalan survives and eats the previously Occitan constituency as Occitan Catalan is unable to power a separatist movement that claims its hypothetical territory outside Catalonia. Catalonia is on the other side of a mountain range after all, naturally limiting its contact with the rest of France and thus partially in insulating it from assimilation, while the rest of Southern France is connected to Northern France through geography, infrastructure and history.
I disagree, the mountain range is a difficult boundary for armies, yet is not so for movements of people. French rule over Catalonia and Aragon reinstates the status quo of the Middle Ages, permitting populace transit through the mountains between the Occitan lands, Aragon and Catalonia. This in turn allows cross influence of political ideology, linguistic empowering and the restructuring of Occito-Catalan political identity and legitimacy.
If one checks medieval sources of this region, you find the mountains were mere highways of people. The crossing from Foix, Toulouse, etc unto the Principality of Barcelona or Kingdom of Aragon, were commonplace. It was even common for men to travel these distances for marriage and business ties. Such freedom of transit and relations is partly the reason the Albigensian crusade was such a fearsome bout and further why the Treaty of Cordeile and later Hispano-French antagonisms were so novel in revoking this historic relation between Occitania-Aragon-Catalonia.