So, can I activate this battle prep just by selecting Mike in the "Dorgania Slaves" quest?
You need to talk with Theremis in the throne room in Aldlyn to "prepare for war"
Ok thanks. If I choose this option, how do I open the scene with Sinrul, don't I have to choose the government for her scene?
Sinrul's scene is available at the (current) end of the rebellion events, regardless of whether you picked to help the rebels or the government.