What are your characters goals? What is he willing to do to achieve these goals? Those are the questions you need to answer to figure out your alignment.
It sounds like you want to overthrow the hose of Thrune and replace it with your own. Are you doing this for the good of the people of Cheliax, or are you only interested in your own power? Also what do you intend to do once you have come into power. If you are trying to overthrow the house of Thrune so you can save the people from evil that has overtaken Cheliax then you would probably be neutral or maybe even good. If on the other hand you simply intend to replace the house of Thrune with your own house and still keep all the devil worship you are more than likely evil.
What methods will you employ to defeat the house Thrune? Are you using any means necessary to undermine the state regardless of who it affects? Or do you work through the system subtly undermining the enemy? Do you operate openly in opposition to the house of Thrune, or do you try to appear loyal while waiting for your chance to gain control. The last part of the puzzle is, are you working mostly by yourself or with a few chosen, or do you have lots of allies that are working together? If most of your actions follow the first examples then you are probably more chaotic, if you follow mostly the second examples you are more lawful. If you are evenly split you are neutral.
Keep in mind that lawful evil characters are usually fairly loyal to those in their own group. They have no problem betraying people outside their group if there are no consequences. Also lower members may be expected to sacrifice themselves for their superiors.
From what you have said I would say your character is probably Lawful Evil.