About the POD you are right, about the new ruler of Bavaria, if Maria Theresia can not rule it of her own, I think is more likely seeing it going to Joseph instead of Francis Stephen. Louis XV can still go out as winner in Netherlands also giving them to the Elector, if he agree to swap under the condition who Max married one of his daughter and Max’s eldest sister (and now heiress presumptive) became the second wife of the widowed Dauphin (putting Netherlands firmly in the French sphere with a pretty good chance to annex it in the next generation)...
The title of Duke of Brabant is unlikely to be used as Isabella and Albert, while independent rulers, had still Spain as overlord so I believe Maximilian will use a new title
Its possible that Maxi MIGHT get away with the one (a marriage to Louis XV's daughters) OR the other. But if Maria Antonia is already taken by the time the dauphin is widowed (especially if Maria Antonia already HAS kid(s) by this point), a younger sister will look like Catherine of Aragon after her mother died.
The other reason for the choice of Maria Josepha of Saxony was personal as much as political.
1) her sister was queen of Naples so it was preserving the pacte de famille between Versailles, Madrid and Naples (which could've splintered due to Louis XV's rejection of the OTL duchess of Savoy)
2) she was a Polish princess and France wanted to AVOID their main ally in the east (this was before the Diplomatic Revolution) becoming a Russian/Austrian puppet (after 1737 they realized Stanislas was a lost causs). Hence why there were talks about marrying Adélaïde to Prince Xaver.
3) her mother and sister's fertility
4) her uncle was the Comte de Saxe, a marshal of France and a personal friend of BOTH Louis XV and La Pompadour AND Marie Leczszynska (who otherwise was NOT thrilled with the idea of this match and her dad had to come talk her around to agreeing to it).
5) France wanted the secret to hard-paste porcelain (Meissen ware) for their own manufactories at Sevres.
To give an idea, there were other German candidates considered for the dauphin OTL (Theresia Natalia of Brunswick, Auguste Elisabeth of Württemberg, Elisabeth of Baden-Baden, Anna Viktoria of Hesse-Rheinfels etc). Elisabeth and Anna Viktoria's causes were pushed by their respective relatives at Versailles (the Orléans and the Condés/princesse de Carignan). Theresia refused to convert and Württemberg couldn't meet the dowry requirements.