Keep in mind that Ferdinand is an absolute master in keeping his emotions hidden and putting up fake emotions that seem to fool anyone other than Rozemyne, and maybe his name-sworn, so it’s entirely reasonable for the royal to not take him at his word, just because he says he doesn’t want to be Zent… not to mention I seriously doubt he bothered to ask Ferdinand, whether he wanted to be Zent. The only thing we know is, that Trauerqual essentially put an ultimatum before Ferdinand: Replace Sylvester as Aub Ehrenfest, marry Detlinde, or get executed. And as we’ve just seen in this chapter, he believed that had Ferdinand made a move for the crown, at that time, it’d have resulted in a civil war and thus needed to be averted. Ferdinand making a more to become Zent after marrying into Ahrensbach hadn’t been thought of either, since it wasn’t an option anyway at the time, and was a path only potentially opened up by the discovery of the magic cricle and the assumptions this lead to.

I very much doubt that Raublut is to blame for Trauerqual’s reaction here. His words are entirely in keeping with what else we know of his true feelings. I also very much doubt that Raublut could have forseen Detlinde’s action and the immediate reaction by the Sovereign Temple, making it unlikely he could have been able to get Trauerqual drugged up enough to alter his mind, between the incident and this meeting. I also don’t really see what Raublut would have to gain from manipulating the Zent here and now.