Speaking of which, and sorry of dragging this too off-topic, but when the US imposed the metal and oil embargos on Japan, what were their requirements, if any, to resume metal and oil deliveries?
On another hand, what is the feasability of switching most of the fleet to at least partially coal fired boilers? They did had such ships apparently during the WW1 years and into the twenties (like Akagi and Kaga), and were trying the same type of conversion in 1945. Is that even feasible? Have i read correctly that Japan did had substantial coal reserves?
Again, sorry if i'm detouring the topic too much.
As to the embargo, the requirement was to withdraw from French Indochina. Effectively annexing France's colonies, not the war in China, as is sometimes believed, was what brought the hammer down on Tokyo. The U.S. had told Japan flat out not to do it, or else. They did it and the U.S. (and it need to be noted, UK/Commonwealth) followed through.
They might have been able to make the conversions, but it would have taken the ship(s) involved out of service for months. There would also be the need to remove fuel tanks and replace them with coal bunkers, change bulkheads to allow movement of the coal to the boiler room, add personnel (and berthing) to stoke the boilers (i.e. throw in the coal into the furnace), and change the configuration of the hull to add coal chutes. The carriers will need their islands to be redesigned, the deck level funnels that worked with fuel oil will be... sub-optimal with coal. Pretty much change the ship completely. The ships will also likely lose a couple knots, and a ton of range, oil is vastly more efficient as fuel source than coal.
The other issue then is that you need a whole fleet of colliers to keep the ships supplied as well as moving massive qualities of coal to the Mandates, Korea, and Formosa. there will also need to be more infrastructure put into place to allow large volume coaling, since warships will burn much more, and much more quickly, than fuel oil. Coaling is also massively labor intensive, increasing turn around time.
Using coal on carriers will also have the potential to serious complicate flight operations due to the quantity of smoke produced while at high enough speed to conduct flight operations.