How sound is the interpretation of "touching" (al-mass) in the Hadith as sexual intercourse, and the interpretation of "a woman not lawful to him" as one who is forbidden to him, meaning one whom he is not permitted to marry?

The mentioned Hadith is authentic and is not a euphemism for intercourse or anything similar. Rather, it refers to the mere touching of the skin of a person who is not permissible to touch. Allah has commanded lowering the gaze in His saying, "Tell the believing men to lower their gaze" (An-Nur: 30). Touching is more intense in pleasure and arousal of desire than mere looking. The word must be taken at its apparent meaning unless there is a hindrance, and far-fetched interpretations lead to emptying texts of their intended meaning.