After several attempts of trying to break into the force of Praven with several thousand (it seems) enemy Swadian lords, I can conclude it is impossible to commit a prison break without seiging. There is no option to enter the prison.
-Killing the prison guard does nothing
-Killing everyone in the 'Gate' scene does nothing
-Having a nearby village start a fire then try to enter the prison using the above methods still does nothing
-Demanding the keys, then quickly trying to enter the prison still does nothing
This has really made me quite salty, as it took me several minutes and tries to get to Praven in one piece, just to be greeted by this.