Is regularly seeing one's deceased father in a dream, appearing young, smiling, and dressed in white, an indication of his comfort and tranquility in his grave, and why have these visions ceased?

What you mentioned about your father's death, that he died with sweat on his brow, and that you saw him in a dream in white clothes, is a good sign. Dying with sweat on the brow is one of the signs of a good ending, and seeing the deceased in white clothes in a dream is a sign of goodness. As for him not coming in a dream again, we have no knowledge about that.