This is more of an opinion more than an answer, but you know when Gestahl pretends to offer peace, and has Kefka put in jail? But he later let's him out and takes him with him to the Sealed Gate, not to mention sending him to despose of one of their own.
So, did Kefka know about all this, and was simply pretending he'd been arrested? i.e. did Gestahl simply tell Kefka to wait in the cell until he was needed as a means to fool the good guys?