How do the Butterball Cenobite's glasses stay on? He has no ears!
They're probably a part of his face.
Pretty much. We are talking about sadomasochisic demons after all. They enjoy anything that causes severe pain, and fusing one's eyewear to their head would be pretty damn painful.
At the end of the first film, all the cenobites get sent back to hell. Except the Butterball, he gets crushed by some falling masonry. But he's a demon, it'll take more that a falling rock to kill him. Sure, it'll be painful but he can obviously deal with that. IDK, it just bugs me.
He's fine in the second movie, so getting "crushed" probably just resulted in him getting immediately de-summoned. Alternately, getting crushed kept him pinned him down while Kirsty kept going through each step of reversing the puzzle-box, and one of those steps teleported him back offscreen.
Why did the Cenobites try to welch on Kristy's deal in the first movie, when they were perfectly fine with sparing her in Hellseeker?
Because as the movies went on the Cenobites suffered an awful lot of character and motive decay, that's why.