Right, so as you can see, my invasion is prepared and I am ready to declare war, but have no option to do so. Choosing the option below and using my prepared invasion troops just results in losing massive prestige. As far as I can see, there is no way to trigger this invasion. Below is the save file.
http://www.mediafire.com/download/i9om5fflc29m0pa/autosave.ck2