On the last part: sometimes words drift to be widely accepted as an exact opposite of the original meaning. I think that happens because they were never popular enough for people to remember what they really meant or because too many people used them incorrectly.

An example you gave “fat chances” feels like it was originally sarcastic but then stuck, “quite a bit” feels the same way although I don’t know for sure.

And then apparently there are also contronyms that has exact opposite meaning, so yeah some things just require more explanation 😅