Your assessment is not far off the mark. There are two things to consider, however: firstly, there had been a strong history in the years prior to the Mukden incident of military officers assassinating government officials that made decisions they disagreed with. Secondly, there was strong public support for the actions of Kanji Ishiwara, the colonel who invaded Manchuria during the Mukden incident. Essentially, the government was forced to either accept Kanji's fait accompli, or risk antagonizing a military and a public that was known for being willing to employ violence against civil public officials like them. Add that the presence of an active-duty army and navy officer on the cabinet was required for any prime minister to rule, and there was ultimately very little that the Japanese government could have reasonable done otherwise.