Does the volunteer fasting person attain a merit other than what he intended, even though he did not include it in his intention, but he performed the action leading to it?

All the intentions you mentioned for fasting are not required of you. It is sufficient that you fast for the sake of Allah Almighty. If Allah accepts your fast, you will find all that Allah has prepared for the fasting person, such as the answering of prayers, the distancing of your face from the Hellfire, and entering through the gate of Ar-Rayyan. And bringing these intentions to mind while fasting does not prevent one from receiving the reward for fasting.