Very true. Even if they wanted to, it would have required the longer end of the time line if we assume that the first landings at Guadalcanal represent the earliest possible attempt to reinforce the Philippines had it held out. But of course that is just an assumption.
I can’t think of anything specific I need at the moment except for some stuff in College Park, Maryland at the National Archive II, but I am writing a major paper on American and Japanese cryptology in the 1930s and early 40s this spring. If you are poking around and run across anything interesting let me know.
There is a very interesting article written by R.J Hanyok which appeared in the Dec 2006, issue of World War II magazine titled "Blinded by the Rising Sun: Japanese Radio Deception Before Pearl Harbor"