None of the triumvirate would be elected; the emperor is the emperor, the field marshall is the highest ranking military rank, and the lead scientist would be the seniormost and most accomplished scientist in her or his field. The latter two would, of course, be appointed by the emperor (Someone's got to pick the next field marshall or decide who out of her or his scientists is the best. So it would be like the three kings of Sparta. Sort of. >.>
I really don't want a triumvirate. As you say, rivet city already has one and honestly, a scientist is not a leader in any way and shouldn't be considered a major political player. Sure, they're useful and their advice is very valued, but they're not
leaders. An advisory council maybe, with representatives of the military, scientists of various fields, agriculturalists, farmers, teachers, etc.
I do see your point about the bureaucracy, I just don't think that 130 years after their empire was desolved and they progressed rather like our japan (as they have always been outside of the major influences like civil rights, the peace and love movement, the ambiguity of the american's generation X, etc. Japan has almost been a bubble nation and I really don't see that being any different in the fallout world. According to the fallout bible, China didn't become aggressive until the US cut them off from oil trading post 2000. It was only then that they became aggressive and invaded alaska to take the oil reserves for themselves. It just doesn't feel plausible that ~130 years of democratic government would just wash away like that. I would sooner believe that the military forces would assume control at first, as the entire group/community/contingent/whatever of Japanese that eventually turns into this kerberos force we're discussing would naturally turn to the military for leadership and not the son of the Emperor. From there I can imagine them eventually forming a type of governing body similar to the progression from US Army to Brotherhood of Steel.
The Empire was never really dissolved. The one term of Japanese surrender in WW2 was that the Emperor stayed the Emperor.
And why doesn't it seem plausible that after 130 years a democratic government would wash away? After the 300ish years of American democracy, we see dozens, if not hundreds of factions fighting for control of what remains of America. The Enclave definitely has the most legitimate claim, but the Brotherhood of Steel does as well - they definitely aren't a democracy. Raiders and slavers only want anarchy and survival of the fittest. Then you see all these little towns that don't give a crap about what government there is, as long as they're safe.
And I really would rather not draw parallels to existing entities in Fallout - creating something original would be my goal in this... being heavily influenced by the Kerberos saga, but not a direct ripoff in any way.
Now perhaps the son of the Emperor was a high ranking military officer who, after several years and promotions, assumes control of the group; but I still don't see him declaring himself emperor as he would have more experience and life as regular army rather than as a living god. By that point, whether or not he was the son of the emperor would be moot because the people will have been acclimated to structure and rule given by the military as opposed to the democratic governments that vanished with the first bright flash on the horizon. Also, the military officers, NCOs and regulars would not immediately turn to him for guidance leadership or anything that overstepped his rank as major, captain, leftenant, etc. By the time the bombs fell it would be like the military in the UK turning to Queen Elizabeth or Prince Charles for leadership instead of the Prime Minister or Field Marshall. By the time the bombs fell in 2077, the memory of the great empire would be just that, a faded and distant memory. The Emperor would be thought of as a figurehead and very little serious thought would be put towards his leadership in such a grave situation, especially since he had never had any such responsibility before then.
I was beginning to understand what you were saying and the very end nailed it dead on. You're thinking in a very Western mindset, where the Queen is just a rich human whose parents were also nobility. The Japanese are not like that. The Emperors of Japan are descendant from the sun god. Yes, they have a lot of human blood in them, but they also have the blood of a god flowing in their veins. Yes, the military would and does look up to the Emperor. In WW2, every weapon the Japanese military used personally belonged to the Emperor - they were on loan to the soldiers. They lived for the Emperor, they went to war for him, they died for him, and in the end, they surrendered because of him. If he hadn't told his people to surrender, they would have fought until the last man, woman and child was dead.
While the Emperor will never have direct military experience, he will have had tutors from an early age teach him about basically every aspect of society, these tutors being among the best in their field. The Emperor doesn't just sit on his throne all day, he has to be one of the most academic people in the world. Of course he'll have a General to lead his armies, as he'd have a Prime Minister to run the day-to-day operations of the civilian sector... but the national direction and objectives, the overarching strategy of the Japanese people, would be decided by the Emperor.