Not sure if this has been asked, and not sure if it was ever a thought. But I noticed that your demonbae Wirt can look frightening. Has he ever done that weird jittery thing the Beast did before he covered the area in darkness? Maybe he didn't mean to do it, and was stressed beyond normal, and did it just to tell others to be quiet for awhile?
So if you have read the amazing story @that-one-girl-behind-you wrote for this universe there is actually one scene where he does that, when he is at the mansion with the Northwest.
So yeah he can do it, but he doesn’t do it much if at all unless completely unavoidable. Wirt is too good of a guy to just go around using it, the only stances in where I can see him going full demonic beast is when someone he cares for is in danger, or in the case of the fic where his own life was in danger.
Hope this answers your question!