As to the question of Mac's replacement, and whether or not I have overcomplicated maters, I think that they are far more complex than would first appear.
First off, it is still peacetime in America, so all the normal protocols will apply. Now I freely admit I am not an expert on the workings of the American Military, or the American political system. So I could be totally wrong, and if I am, then feel free to tell me.
Now my understanding is that America has in-place curtain regulations in regards to signor military appointments, and these include Senate confirmation of such appointments. Given that such confirmation is a political matter, Gen Marshal will need the Secretary of War to send this matter to the Senate. He the SoW, would expect to be consulted before being asked to advance his recommendation to the Senators, as would the President who I believe is technically the the C in C of American military forces. And would simply as a matter of curtsy and good manners, to be consulted.
There is also the question of the USN, who if the replacement for Mac, is also commander US Forces Philippians, and the Asiatic Fleet is included. Would also expect to have their opinion sort out. While the Philippians has not yet been granted its independence, it does have an elected government, which includes a President, and he and the local political establishment would expect to be involved in the question. As will the American equivalent of the British Foreign Secretary, who given that the Philippians are in the mix, will want to be included.
There is also the question of legitimacy, if there is a requirement for the appointment to be confirmed by the Senate. Then if for any reason the man that Gen Marshals puts in place hasn't received confirmation from the US Senate, he can not take command and issue orders. Any officer would be perfectly within his rights, and to an extent required to ignore them.
Once again if I have gotten it all wrong, then please put me right.
i
RR.
Your point that it is peacetime is key. So what likely happens is Wainwright fills in as acting commander while a formal replacement for MacArthur is selected and then gets through the confirmation process and all of that other great stuff. This all probably takes some time and the war probably starts before MacArthur's replacement gets out there so Wainwright becomes the commander by default.
For a timeline consider this - Kimmel was formally relieved of command on 17 December 1941 and Nimitz did not arrive until 31 December even though he was notified that he was Kimmel's replacement on the 16th and that was after the war started and Hawaii is a lot closer to the US than the Philippines.