Was Watanabe's Midway planning already underway at the time of the recent bombings of Marcus Island by U.S. Navy planes?
Yes and no. Conceptual work concerning an attack on Midway had been underway since mid-January when Admiral Matome Ugaki began studying what the very successful Japanese fleet should do next. He came up with a plan to invade Hawaii which required preliminary invasions of Midway, Johnson and Palmyra Islands in June. This plan was promptly rejected because (1) they couldn’t expect to achieve surprise on Hawaii again, (2) there was not the resources required to achieve complete air superiority over Hawaii, and (3) they would have to contend with Hawaii’s formidable shore batteries.
So the Midway/Hawaii idea was shelved and planning was conducted on an eastward attack. By late February this plan was complete and presented in mid-March. However, the plan called for operations against Ceylon and required a joint effort with the IJA. The army rejected the plan leaving the Combined Fleet adrift as to what to do next.
So Ugaki’s Hawaii plan was pulled back out since the IJN was concerned about the problem they were having with the U.S. carrier raids. It was decided that the Midway portion of the plan would create the best possibility of drawing out the American fleet. The preliminary operational plan that would be presented by Wantanbe was drawn up on March 29th and 30th for this April 2nd presentation. Debate on this would go on until April 18th’s event would end discussion on the matter.