Because it's quoted speech, it does not need to follow grammar. The character is speaking in fragments. However, without reading the book, it's hard to understand the full context of this situation. The paragraph you linked is helpful, but there's nothing to tie 'does' to the previous words. So, "Never does." doesn't make sense to me. It could be, "It never does work out the way you want it to." but I'm just guessing. So, here's my best guess: "Don't say it. It never does work out how you want it to. You'll wish you hadn't later." All of this is referring, I guess, to "make myself useful" but I'm not 100% sure. Sorry, just remember that natives struggle with literature at times, so don't be too hard on yourself. Sorry I can't help you more.

It's really hard to tell without context. Do you have the paragraph, or a link? Even the name of the book would help. It looks like the colloquial style of removing pronouns and other parts of the sentence. This is very informal, and not something we do often. I even suspect "Wish you hadn't later" is supposed to mean "You'll wish you hadn't later."