Regarding your questions: Once prasadam (water, or foodstuffs) is offered to the Lord, His Plate should remain on the altar for about 15-20 minutes (while the portion in the pots may be taken by the devotees). Water need only remain there for a few minutes. Once the prasadam is removed from the altar, and touched by us, it should not again be placed on the altar. Whatever we offer should be as fresh and pure as is possible.
So long we are engaged in the service of the Lord, He has assured us all protection. But if we are attacked, of course we must defend ourselves as best we can. Nobody can compare with the Merciful Nityananda. He can defend Himself quite well; He could have killed the sinners in an instant, but He had decided that He wanted to save these sinners. He was setting an example that we have to preach Krishna Consciousness even at the risk of personal violence. Just like the children sometimes become violent, but the father knows he can defend himself at any time, and so he tries to teach them correctly by reprimanding them.