given what the Delvian sect are willing to do to the Moya crew without crossing what they see as the moral line by using physical violence

Indeed. When JMS let Psi Cops do stuff like this in early B5, it was to signal these were the bad guys.

Tuzak: now that you mention it, yes, there are no scenes where he comes across as actually insane, or even sane and dark. (I wonder whether the Writers has something like Stark in "my side, your side" mode in mind?)

Or: Maybe Tahleen manipulated him into appearing insane to the other Delvians by her superior mental abilities?