Greetings! I will attempt to shed some light on the interesting and somewhat perplexing situation of the Incident, and I must give all due credit on this fairly under-researched (at least in the mainstream source areas) topic of Japanese interwar history to historian Stephen S. Large (whose article is linked below). Even with his extensive writing on the matter however, I am afraid there is next to nothing about the lives of these assassins once they had served their greatly-reduced sentences. A minor nitpick however before we begin, the assassination of Prime Minister Inukai Tsuyoshi occurred on May 15th, 1932. Hence your question is off by two days, but it is a crucial error. Let's begin then.

The 11 assassins involved in the May 15th Incident were mostly army officers supported by navy counterparts and some civilian members. All of them were members of right-wing ultranationalist groups, including the notorious Ketsumeidan*,* or "Blood-Pledge Corps", which had staged their own wave of violence against high-ranking business members and government officials in March.^(1) During the trial, the public mood was completely in support of these bold young men who had acted in the interest of the Emperor and the glory of the Japanese nation. War Minister Sadao Araki himself spoke highly of these noble acts, as seen below:

We cannot restrain our tears when we consider the mentality expressed in the actions of these pure and naive young men. They are not actions for fame, or personal gain, nor are they traitorous. They were performed in the sincere belief that they were for the benefit of Imperial Japan. Therefore, in dealing with this incident, it will not do to dispose of it in a routine manner according to short-sighted conceptions.^(2)

With such a high-ranking statement, the public mood was completely against the prosecution, and a torrent of leniency requests came in to the courts during the trial. Their sentences were light, though we do not have names for all of them, here's what Stephen Large'sarticle can tell us:

• Four years in prison for the eleven young army officers involved,
• Navy Officers: Fifteen years each for Koga Kiyoshi and Mikami Taku as the ringleaders. Only ten years for Yamagashi Hiroshi,
• Civilian Members: Life imprisonment for Tachibana Kosaburao. Fifteen years for Okawa Shumei
• Ketsumeidan members: twelve years for Kawasaki Nagamitsu, eight years for Horikawa Hideo, and five years each for Kurosawa Kanekichi and Terunuma Misao.

All of these sentences were eventually reduced, and some were eliminated entirely as the national and governmental mood swung decisively towards ultranationalist and imperialist sympathies.^(3)

Hope this brief bit of information sheds some light on the query, and I shall now take the opportunity to shamelessly plug my four-part response on interwar Japan's political situation, incase you want a deeper understanding of the context in which this Incident occurred.

Sources:

[1]: Large, Stephen S. "Nationalist Extremism in Early Shōwa Japan: Inoue Nisshō and the 'Blood-Pledge Corps Incident', 1932." Modern Asian Studies 35, no. 3 (2001): 533-64. Accessed January 10, 2021. http://www.jstor.org/stable/313180.

[2]: Quoted in Large. ""Nationalist Extremism in Early Shōwa Japan: Inoue Nisshō and the 'Blood-Pledge Corps Incident'"

[3]: Hanneman, Mary L. Japan Faces the World, 1925-1952. Harlow: Longman, 2001.