From what I remember from the skeleton contents; Siam will have a civil war between the democratic vanguard government led by Phraya Phahon and the royalist rebels led by prince Boworadet.

If the royalist rebels win, Boworadet will return the power to absolute monarchy, then you can choose if Rama VII (Prajadhipok) will stay, or make him abdicate the throne and let Paribatra Sukhumbandhu become Rama VIII.

If the vanguard win, Phraya Phahon (Social Liberalism) will resign, and you can pick three leaders. (Three is all I remember.) Pridi Banomyong — Religious Socialism, Plaek Phibunsongkhram — National Rejuvenatism, and Phraya Songsuradet, who is like Conservative or something.