If state governments are involved with the opening dates, the lawsuit would have no merit. Disney has not announced an opening date for either park, and the states have not announced their exact recommended dates yet either, but it is no coincidence that WDW is planning on opening before DLR and Florida is opening before California. It currently looks like Florida will recommend an opening sometime in June and WDW is shooting for July 4th. That is a reasonable timeframe for Disney to say, "We wanted to open as soon as the government would allow and necessary procedures were in place to comply with the new regulation." DLR would only be closed for the remainder of the year because Newsom has hinted he may not permit large gatherings until 2021, in which case Disney would dismiss the argument of not acting in the best interest of the shareholders because they were legally not allowed to open the parks.
This seems to mostly align with the order of the parks in decreasing total daily attraction capacity (MK, DHS, DAK, Epcot I believe) which would help spread out guests.
It really depends on the state and the quality of the lawyers on both sides. The warnings usually do a good job of limiting liability, but they often do not eliminate it entirely.