Re: Hawaii was not an easy target, by any means
Imperial Japan had the troops, certainly. But they were mostly in China, haring after KMT & Communist forces, & desperately trying to find a government that would surrender. They never did, & a large part of the IJA sat out the war there, until the culminating battles with the USSR.
& besides, the whole point to Pearl Harbor was to take the US Pacific Fleet off the board while IJ prosecuted the war elsewhere. Additionally, an out-&-out invasion of Hawaii (if successful) would have involved atrocities against citizens of US territory, lots of US military & their dependents. If that had happened, the US might have dropped nuclear weapons & firebombed all of IJ, not just the worthwhile military targets.
OK, first of all I can see your problem in not thinking of everything that Japan had and over-reliance on future events and developments.
While indeed the IJN and the IJA were often each other's worst enemy, they would also work together very efficiently if the Emperor (or his intermediaries) ordered them to do so. Most of their infighting dealt with resources and who was in command during operations, not in actually prosecuting the war itself. This for example is why unlike in the US where you had the Manhattan Project, in Japan you had Program Ni-Go under the Army, and Program F-Go under the Navy. Both trying to create an atomic bomb, both working separately in competition with the other.
And yes, the IJA did indeed have the forces needed to invade Hawaii, and did not need to tap into the forces in China. It must be remembered, that they invaded the Philippines with 10 Divisions, over 130,000 men. Taking half of them would have slowed the Philippine invasion, but also would have locked up Hawaii, and that was more than enough forces to have secured the main island. These were actually part of the pre-war plans of Japan, but it was discarded, as the Command Staff was still thinking the destruction of the Battleships and occupation of the Philippines would drive the US to the bargaining table.
And yes, the US actually did firebomb every city of military importance. Tokyo, Osaka, Nagoya, Kobe, Omura, Oita, Tachirarai, Kagoshima, Shizuoka, Koizumi, Tachikawa, Kawasaki, Yokohama, Iwakuni, Toyama, Kure, the list of cities leveled by firebombs god on and on. By the end of the war, few cities of over 100,000 people had not been devastated by these attacks.
And you are also forgetting how the Invasion of Hawaii (even ultimately unsuccessful) would have had on the progress of the war. To begin with, any Japanese forces would have to be drawn out, probably delaying the island hopping campaign by at least a year if not more. And of the Battleships needed for that campaign, 5 of them were still in Hawaii. Imagine the effect of their being destroyed by occupation forces in place. Or even worse, salvages and put into service with the IJN.
And 5 Divisions would have likely been enough to occupy the main island. The US only had 2 fragmentary divisions there at the time, the half strength 24th and 25th Divisions. 5 Divisions would have had no problem driving them from their bases, and eventually hunting them down without resupply in the mountains.
And with such on the plate of the United States, what would the effect be on the Manhattan Project? I doubt it would progress as it had in our timeline, with the next obvious step being invasion of the US itself. Odds are it would have been put on the back burner, probably not being completed until 1948 at the soonest.
And the loss of 5 Divisions would not have seriously impacted the invasion of the Philippines either. They still would have been able to wipe the Army from most of the islands, it would have simply taken longer to do so.
