I think these questions should be viewed more or less separately.

The Japanese army was disproportionately, but NOT exclusively, officered by men with samurai ancestry and the lower officer ranks had a majority of middle class families by the mid 1930s. The Imperial Army, which rested control of the country from the shogunate in 1868 was at the beginning totally dominated by Officers who had been Samurai from Choshu and Satsuma domains. Between 1868-1912, twenty out of the thirty one officers who became full generals were from those two samurai cliques. Choshu in particular tended to dominate the very top of the military hierarchy. Lower ranks were dominated by Samurai although they were from all over Japan. In 1890 about 60% of officers had Samurai backgrounds, and 40% were commoners.

In the 1920s this was changing, with middle class backgrounds increasingly common in the lower levels of the officer corps. As a reserve system increased the size of the army, Samurai background declined to about 20% of the Officer corps. The highest levels of the Army were still dominated by the sons of former officers, who would have been likely to be from samurai families, with 35% of 1910 academy graduates (who would have been among top army ranks in WWII) being sons of military officers. The Army's top brass, and pivotal figures in the 1920s like Gen Araki Sadao, (more on him later) and later Tojo Hideki were descended from Samurai families. The major modernization of the Taisho period (1912-1926) was that the Choshu and Satusuma cliques were broken up, and the army's top leadership roles were more open to officers from other areas of Japan although few key roles were filled by men with a middle class family background.

However, and I want to stress this, It is wrong to view the Imperial Japanese Ideology as a survival of Samurai values or a result of the upbringing of its officers, it was instead deliberately inculcated by the military and political leadership in the decades before the Pacific war. During the Meiji Era (1968-1912) modernization was heavily emphasized in the army and aristocracy. For example, european style swords and sidearms were introduced. Titles like Baron or Count replaced Japanese aristocratic titles. In the Taisho Era (1912-26), this trend reversed itself. The threat from the European powers began to be interpreted as ideological as much as military. Democratic and especially communist ideology produced a reaction in the Japanese military. Men like Araki Sadao came to power in the late 1910s and early 1920s by dominating the educational infrastructure of the Japanese Army. They used their platform in the military schools to emphasize spiritual values and extreme nationalism during training throughout the army. As Japan's army expanded it became a force of two year conscripts and reserves, and thus the army training system became a means of disseminating these values into the general population. The "Samurai-ethos" was not something that sprung naturally from the population or from Japanese history, it was imposed by the military leadership from the top down.

Source: Drea, Edward "Japan's Imperial Army: Its Rise and Fall 1853-1945"