Does divorce occur if he says, "If I knew you smoked, you would be divorced," and then the wife smokes with her husband, given that the intention when uttering the phrase was unclear, oscillating between a threat or an intention of divorce, and between smoking with the husband or with friends?

The wording you used implies conditioning your wife's divorce on her smoking shisha. The majority of scholars hold that divorce occurs when the condition is met, regardless of whether the husband intended the divorce or not. However, Sheikh al-Islam Ibn Taymiyyah held that divorce does not occur if the husband did not intend it, and he is obligated to offer an expiation for a broken oath (kaffarat yamin). If you did not have a specific intention, the outward meaning of the wording is upheld, so if she smokes shisha, she is divorced. Any doubt regarding the intention of divorce or mere threat does not affect the view of the majority. We draw attention to the impermissibility of smoking, and that a woman engaging in it is even more reprehensible, as well as the danger of taking lightly the utterance of divorce pronouncements.