Kurloz and Meulin were napping together when Kurloz dreamed of Lord English, Gamzee, and himself. The sound he made afterwards was probably the same sound English made, and I think it was intentional. He had to somehow incapacitate Meulin so that he could manipulate her into doing his/English's bidding more easily. He also chewed off his tongue and sewed his lips shut to ensure he couldn't slip up and tell anybody about Lord English and his plans.

Oh yeah, and said nightmare was probably caused by Gamzee, somehow - remember how he fucked up the human session? Inserting things into people's dreams?

Mituna's last premonition was exactly the same thing Kurloz saw, but without the religious significance behind it, all he saw was a green skull-faced demon with flashing eyes leading two trolls who looked very familiar.. He told Kurloz about it, panicking, and his moirail showed up to 'pacify' him.

Except in this instance, replace 'pacify' with 'bludgeon in the head until he's got severe brain damage'. Now Mituna doesn't remember what he saw, can't tell anybody about it, and as a bonus he doesn't remember that it was Kurloz who did this to him.

To hide what he'd done, Kurloz spread the story that Mituna has burnt himself out saving everyone from a horrible threat. (Don't ask me how he did it if he couldn't talk, I have no idea. Miming it to Meulin?)

....this is probably not so much 'headcanon' as 'actual canon', but whatever.