Yes it was felt that Yamamoto would be safer at sea.
The army had built up a tradition whereby less senior officers would carry out acts with the (sometimes even blatent) encouragement of the Generals in order to prevent any real control being exerted over the army. Sometimes the perpetrators would be punished and sometimes they'd be promoted. Go figure!
He seems not to quite comprehend the Japanese naval position at the end of the war.
In even clearer terms: by August 1945, the IJN had been totally and utterly annihilated. Aside from a few smaller surviving ships, all of the major combatants, aside from the damaged Nagato, were sunk or knocked out of action.
IIRC he was threatened because he was against the tripartite pact and war, so the army had junior officers roaming the country trying to murder him. the IJA was really berserk in the 30's taking the laws into their own hands and no one apparently could stop these hotheads.
The blame doesn't rest completely on the IJA. There was a very important faction of IJN die-hards that had been itching to fight the US and UK for years before Pearl Harbor. The power struggles within the IJN were ruthless and on going until the very end of the war. In the simplest of terms, I'd say the IJA put Japan on course to war, and the IJN smashed on the accelerator to take them there in 1940.
Well I'd firmly put the blame on the IJA. The invasion of China - their idea. Taking Indochina (which resulted in the US oil embargo), their idea. Strike South, well the IJN can take some of the blame there.
It's not so clear cut. The navy had been adamantly against naval treaty limiations. The navy was the one bombarding Shanghai in 1932. Sections of the IJN brass pushed hard for, and eventually got, their own big piece of the pie in China with the South Chinese theater, with Hainan, Amoy, Canton, etc. The navy saw the Chinese war as a good excuse for securing a bigger share of the budget for themselves. The Indies were the dream of the IJN and pushed hard for it, but whenever the promise of a more money for the navy budget appeared, they changed their stance to support it (Tripartite Pact, Indochina) and their haste to put the fleet on full mobilization was a serious step towards kicking things off. The navy was on a war footing for quite a while before the US embargo.
And not to mention the fact that it was the navy, with its chronic oil problems, that did more to shift the focus of Japanese strategy south, when the enemy that the IJA and IJN had been planning on fighting was the Soviet Union.
Their support for movement into Indochina and China led to completing the self-fulfilling prophecy that war with the US over an embargo was inevitable.
Don't get me wrong, the IJA was hugely influential and I think there's no denying that they had the lion's share of political power. It just irks me a bit when I read books about the Pacific War that tend to portray the IJN as having been drawn into the war unwillingly out of fear or the US or admiration of the British. They were no angels themselves by any stretch.
20th AF uses 160 B-29's in 1,525 sorties to lay 12,135 Mines around the Straights of Japan
15 B-29's shotdown 293 Japanese ships sunk 35 of 47 Japanese convoy routes had to be abandoned. Shipping through Kobe down 85%
After the war, the commander of Japan's minesweeping operations noted that he thought this mining campaign could have directly led to the defeat of Japan on its own had it begun earlier. Similar conclusions were reached by American analysts who reported in July 1946 in the United States Strategic Bombing Survey that it would have been more efficient to combine the United States's effective anti-shipping submarine effort with land- and carrier-based air power to strike harder against merchant shipping and begin a more extensive aerial mining campaign earlier in the war. This would have starved Japan, forcing an earlier end to the war