The interwar bakufu came about because the parliamentary system (like pretty much everything in Meiji Restoration Japan) was based on trying to mirror the German system only with modifications to avoid its known failure modes. One issue was that the Prussian civil service and military tended to capture the electoral system for their own purposes, and so government employees, which included the military, civil servants, teachers, and even postmen were barred from voting.

They didn’t like this so they went about trying to find non-electoral means of capturing the government. There was a government workers’ general strike/insurrection that was put down, but one of the most successful tactics was that the Minister of War was required to be a sitting or former flag officer. The military was small enough that that was a limited pool, and given that they controlled the promotions systems they chose their successors, and eventually the military-supremacy types got to a point where in order to form a government the parliament was required to install one of them and keep them happy lest they resign and force a change of government.

The final straw was basically the military deciding to invade Korea on their own initiative and daring the parliament to not support “our boys overseas”. The parliament blinked and that was that.

The imperial line breaks at least twice, even if you don’t count the mythological emperors and assume that emperors are in fact the trueborn sons of the previous emperors that they’re claimed to be. At one point the line schisms in pope/antipope style and iirc the following line descends from the one with the weaker claim.

In any case the emperor has for most of this time been a ceremonial religious front for the actual power in the realm, which has variously been the head of dominant clans, a parliament, the two bakufus (three, if you count the system established between the two world wars where the military subverted the parliamentary system that ruled in name of the emperor), and for one interesting stretch the retired ex-emperors.