1- During this period of time Japan became an established liberal democracy. They also developed the modern party system where business leaders played a heavy part on politics.
2- General Yuan Shikai attempted to establish a dynasty by declaring himself the emperor of China, however after his death loads of local warlords fought for the power and it ended with economy collapse and famine.
3- Social order was threatened by local warlords therefore the politic party Kuomingtang and the communists began the northern expedition however due to a conflict of interests between the members the civil war begun.
4- Because of the national security they promise. Also, because they've got the upper hand in regards to military power
5- Because as mentioned in answer #3 The Kuomingtang united forces with the Communists against Japan as they once did with the local warlords in order to protect their country against the external threat
Hope this helps!
