Only 22 of the army's 1500 generals committed seppku during the war, and only about 600 officers in total did so during the war (just some fun facts). The process of Seppku was used for for those who had dishonoured their name and their family by being defeated; in Feudal Japan it could be used as a capital punishment, or as a punishment on a defeated foe. Hirohito was not defeated, his generals were. Hirohito, according to tradition, wouldn't have committed seppku because he didn't fail, those under him did.

Also it should be noted that Hirohito went to great lengths to make the "surrender" as honourable as possible and to deny his involvement in the war. The speech Hirohito gave to his people, after accepting the terms of surrender by the allies, didn't even contain the word surrender. It was also considered taboo in Japan to talk about Hirohito's involvement in the war. So, Hirohito was able to, in a way, successfully distance himself from the war and the war crimes.

As for you other question, as far as I know, no emperor has committed suicide, but if wait for another Japanese expert.

Sources:

A history of japan by L.M. Cullen

Embracing defeat by John Dower