The loss of castling rights is relevant to 3 fold repetition, but not to the 50 move rule.
For pawns to pass each other, they have to capture pieces, i.e. a pawn move and capture at the same time.
It is multiples of 50, not 49. White and black move knights. On move 50 when it would otherwise be a draw, white moves a pawn or captures a knight. Then they move pieces again, until move 100, when a pawn has to move or something has to be captured and so on. Ultimately when a king captures the last remaining piece it is a draw.