Ha, one of the canons I'm writing is Homestuck, so sex tentacles could be proliferating like flowers in springtime. As for it being detachable, why not? There are weirder sex toys in canon. Even though I'm not planning on sex in that story, I do love me some bedsharing, so theoretically a sex tentacle could go wandering for some dubcon; perhaps in a slight AU.

As for the other, that would be entirely the wrong kind of body horror to have. The protag's just going to be coughing up gouts of blood instead of getting extreme body modifications!