How did montresor know that none of his servants would be home

The servants do not "abscond" because Montresor tells them to stay at home; they do so because he tells them he will be gone all night. They don't care if he finds out that they all left to join in the carnival. He can't fire them because he can't get anybody to replace them.

onto a large boat, and mark the water level that the boat sinks to. then, take the elephant out and put a bunch of marked weights onto the boat until it reaches the same water level. add up the masses of the weights to get the weight of the elephant.