There is an excellent biography of Yamamoto called 'The Reluctant Admiral' which shows that Yamamoto certainly had the means, the motive and the timing. The supposedly secret plan to attack Pearl Harbor was leaked via Argentina almost a year before. The US Secretary of War knew Pearl Harbor would be attacked and when, more than 24 hours before it happened, and thus was more than ready to address the (late!) Japanese envoys when they came to his office. The Japanese secret diplomatic codes had been 'broken' for several years, and all these things show up just shortly after Yamamoto had been on a mission to the UK, where he had been sent by the radical Japanese Army that had taken over the government and either assassinated or deposed some of Yamamotos best friends and mentors. In the UK, Yamamoto spent a significant period of time at the house of, and by invitation by the head of British Intelligence, and they had many long discussions (still secret). However, when Yamamoto was on his way home, he and the other members of his mission stopped off in Germany, supposedly to conduct the same sort of negotiations. But Yamamoto refused to meet with Ribbentrop, or any of the German representatives, and thus went home to Japan without anything significant done on his part in, or with Nazi Germany.
Yamamoto was well-known as a gambler (he even considered quitting the Navy at one point to become a professional gambler at Monte Carlo!), he was also a womanizer, with several affairs going on at once, was extremely angry at the deaths of his friends, comrades and mentors in Japan, and was something of an alcoholic as well. All of these things constitute 'leverage' to intelligence personnel looking to subvert or suborn, and certainly Yamamoto was a perfect candidate for such activity!
During the war, virtually every Yamamoto battle plan involved the gathering of widely dispersed small units travelling over widely dispersed approach routes to a central assembly area which would supposedly be the objective. However, this 'strategy' ensures that not only will the different elements arrive at various times, but the likelyhood of any, or all of these elements being discovered was quite large, each of which could be dealt with by locally superior forces (as against keeping the fleet together, which is just one needle in the haystack of the Pacific, vs a whole box of needles sprinkled throughout). And this 'strategy' failed again and again. Even in those occasions where it was successful (first six months), this was almost entirely the result of the incredible disorganisation of the allies combined with local Japanese air superiority, and not because of any particular Japanese tactical or strategic superiority. Furthermore, after Yamamoto was killed, his replacements largely continued the same failed tactics, with the same fatal results.
One particularly nasty trick of Yamamoto was to 'support' a landing somewhere (like Wake and the various Lae and Guadalcanal reinforcements), but to always ensure that the Army units landed would be commanded by some of the most radical elements in the Japanese Army, the same lunatics that had seen to the assassination of his friends and mentors before the war. Yamamoto would ensure that these units would be far too small to possibly take on the defending Americans, and that they wouldn't have near the supplies and equipment to make anything like a defense either. To a man, these units, and their commanders were slaughtered wholesale in one stupid 'Banzai' charge after another (the first of these, the 'Iki Unit' was originally slated to be the landing force at Midway, but after that failure was sent straight in to Guadalcanal after the landing by the US 1st Marine Division. That night, the entire 'Iki Unit' of some 800 Japanese soldiers led by their commander charged the lines of the 11,000 man-strong 1st Marines, and were unsurprisingly completely destroyed.).
One incident of this type could be overlooked as a tactical 'mistake,' but to have the same thing happen again and again in the same place for six months says something else entirely! Another indicator was the apparent lack of ability to bring in sizeable reinforcements and their supplies. It would take the famous 'Tokyo Express' days, or even weeks to get a few hundred troops ashore at Guadalcanal, and yet, at the end of the campaign, the same destroyers were able to get the entire Japanese force of some 14,000 troops off the island and back to Rabaul in just seven nights, and without loss! Not only that, but the commander of the amazingly successful 'Tokyo Express' was then summarily sacked, and sent to Thailand to do a useless desk job (Admiral Tanaka never went to sea again). How could this make any sense, without some significant skullduggery going on at the highest levels of the Japanese Navy? Read the 'The Reluctant Admiral' and see how many peculiarities you notice!