There is a possibility that it is unconstitutional to for a State to determine its method of appointing electors via the initiative process.
Article II, Section 1 says: "Each State shall appoint, in such Manner as the Legislature may direct, a Number of Electors ..."
The SCOTUS would have to determine whether "Legislature" is the body that meets in Sacramento, or whether it represents the "legislative process" of California. The principle behind the initiative, is that all legislative power is vested in the People, and that the legislature only acts under a conditional grant of authority. In California it might also face a court challenge because California does not permit the legislature to modify an initiated measure without a referendum. Article II, Section 1 could imply that the legislature of a State could change the manner of appointment at will.
Depends. Some states that use the initiative process explicitly state in their constitution that the people as a whole are effectively a third house of the legislature. However, the California constitution makes a distinction between the Legislature and the people. Thus if a case were to be made, I suspect that the Supreme Court would first ask the California Supreme Court to rule on whether they consider the people to be a legislative body under the California Constitution.
Another consideration is standing. I can't see a case being accepted until such time as the California Legislature attempts to change the result of an initiative.
Finally as an aside, has there ever been a case where a Governor vetoed or attempted to veto a measure passed by the Legislature that specified how electors are to be chosen? Such a case if has occurred would have considerable value as a precedent.
