Is the finale mission determined by the result of the last story mission, or is it chosen from available side missions that are in play? If it's the former, then is it even necessary, at the end of mission 9, to place a new side mission in play?
How To Determine Final Mission?
Yes, it is decided by the results of Story Mission 5, and yes that does mean that you don't have to bother to put out a new side mission after it.