What did you want Fourchenault to do? The entire royal family is dead except for Zenos. Zenos has been exiled, doesn't want to rule, and would clearly be horrible at it. Gaius refuses to set foot in the country because half the population wants him dead because they think he killed Varis (which means he can't effectively govern them, because he doesn't have the military strength to force them to obey). The only Garlean candidate I can see for an actual monarch at that point is Vergilia van Corculum, who at least still has the Third Legion, and she really doesn't seem like she wants to be empress. "Well, you used to have a Senate, right? Let's have the surviving members run things while you decide how to put a government together" seems reasonable enough.