As a Chinese receiving education on history in Hong Kong, but with one eighth of Japanese blood, I would still argue that it remains difficult to prove the responsibilities of Emperor Hirohito during the war. I have never been an apologist of war crimes committed by the Imperial Army, but to say that the Emperor should be held responsible would've doubtful. The Japanese Emperor was given so-called strong power and authority since the 1868 restoration, but his power was exercised by the Meiji oligarchy, later the genro. The emperor, after all, was just a symbolic figurehead.
Before the death of Kinmochi Saionji in 1940, as the remaining genro in a new era, Emperor Hirohito indeed appointed every Prime Minister on the advice of Saionji. From 1928 to 1932, Saionji would recommend the majority party leader or the one who could command a House of Representatives majority to become Prime Minister. In case the party government fell, he would invite the opposition party leader to become Prime Minister, and advise the Emperor to call for an early election as soon as possible (every government fell apart from 1928 to 1932). Then, it was Saionji's decision that parties and civilian control no longer worked after the infamous May 15th Incident, so that the Emperor should no longer appoint the majority party leader, leading to outrage and backfire from the traditionally reliable Seiyukai establishment.
Saionji himself would be targeted in the February 26th coup attempt, the emperor's opposition to the coup was also a direct result of Saionji's strong dislike of the totally insane Imperial Way Faction that would have led Japan to the path of destructions. Did the Emperor have power to stay no? Technically yes, but he had never said no to a genro ever since the late 19th century. Unrelated to the war, since many would agree that for one day Saionji was alive, Japan would never try to bomb thr Pearl Harbor, this is to show that the Emperor did not really have a say.
After the death of Saionji, the last check of the Emperor's power was gone, but it was indeed just a check of army and navy power, because power did not exist at all on the hands of the Emperor. When you look at historical documents like the Emperor's journal or diaries, as late as in 1940, the Emperor had doubts if Japan could really win such a war, though he would at one time become a little bit more optimistic.
Who was in charge from 1940 until 1945? It was not the rule of a single person. You cannot even possibly say Hideki Tojo was behind, because the Army and the Navy as a whole ruled as a joint leadership, where the Emperor would just have been informed. To say Hirohito should be held responsible for war crimes, would be like accusing King Victor Emmanuel III to be a war criminal. Japan was under the rule of a junta ruling in the name of the Emperor. A monarch normally should not intervene no matter who's in power, what he had to do was to accept the decision made by the ruling junta - that he was Head of State did not mean that he should be held responsible.
As for the monarchy question, if the monarchy was to be abolished and Hirohito tried, expect Japan to become a failed state that would not ever rebound even with the Korean War. Open revolt or even assassination attemp against the collaborators or SCAP leaders including General MacArthur would likely have taken place. If anyone at the time believed that the Imperial Household has lost credibility, it would be easy to have a referendum on this topic, and the monarchy would be kept by comfortable margins, if not a landslide. Without the Emperor's symbolic existence, the SCAP would have been a total failure. Competent civilian leaders like Shigeru Yoshida may have been assassinated in a wave of assassinations much serious than what we saw in the early 1930s.
So how about the abdication of Emperor Hirohito? It would not work at all. A new Emperor would not have given MacArthur the cooperation and support of the Japanese people. If Hirohito was to be tried, the trial itself would create extreme reactions from nationalists to reactionaries across Japan. That means, what we see now is already the best possible way to deal with issues, like it or not.
The Emperor committing suicide would not work as well. You may well have prevented a wave of assassinations, but many in the elite class would feel bounded to commit suicide likewise, or to lose all hope instead of wanting to cooperate with the Americans. The transition process would be equally painful. Expect the suicide of a fifteenth of the population, and a deeply divided society impossible to be governed.