I think that Max Emanuel would only get involved if there was something in it for him, basically he'd behave like he did OTL, claiming that he had some right to a piece of the Spanish Netherlands per his marriage contract to Maria Antonia. He's never going to be happy with Charles on the Spanish throne, he really believed the Austrians were behind the death of his son, so he'll do whatever he can to throw a wrench in their plans especially if he sees an opportunity for himself to profit at the same time. The French would probably be fine paying lip service to the integrity of the Spanish Netherlands while giving him a nod and a wink when he suggests that he keep the Austrian army out if he be allowed to annex some piece of it. And technically he's not fighting the King of Spain if he's standing in the way of Austrian troops, and if Charles III tries to depose and replace him as governor he could threaten to defect outright to the French camp (even though actually doing so would probably drag England and the Netherlands into the war) so unless the Austrians want to risk calling his bluff they may just have to put up with it. Besides without sure support of Britain a campaign in the Spanish Netherlands is just one more front when the Habsburg forces are stretched thin as it is.
As for the Austrian succession I agree, if the Spanish Succession is not settled quickly it will become an issue and complicate the war a great deal further. And Aphrodite makes a great point about the Great Northern War. If England and the Netherlands are not fighting France they could possible get involved in that conflict.