This is a very awkward sentence to make a contentious statement.
An act, plan, plot, riot, etc is 'instigated', but a mob would be aroused, fired up or 'egged on' ie encouraged. The suggestion is that the agitators were earning or hoped to earn 'a comfortable living' by their actions, and that they were not able to do so by other means. 'Otherwise' means 'by other means', 'in another way'.
'The bourgeois mythology' is the way in which the upper middle class of society viewed the working class and the 'agitators'; 'mythology' implies that it is a false impression - a myth.