Is a pilgrim sinful for taking from Zamzam water he found near the elevator after his companion convinced him it was for public use, and does that affect the validity of his Hajj?

You are not sinful for taking what was placed in front of the rooms, because it was most likely placed there with the intention of being a gift. The basis of most Sharia (definition) rulings is the غالبة الظن (preponderance of probability). Even if it was not intended as a gift and had an owner, your taking it does not affect your Hajj.