BPing Arrows take time, read the bridge scene, Archer was taking several seconds to charge his arrow and shoot, that was why Shirou got that time window to get saber to jump and project his shield. Remember that Archer's normal firing speed is similar to the rate of a machine-gun, spamming arrows continuously at the enemy.
Shirou can dispel his own projections, that is right, but I do think if Lancelot ever took over one of them, Lancelot would become the owner of the projection, and thus Shirou would not be able to dispel it anymore. Also, I am willing to bet, that with some good luck rolls, Lancelot could catch a BP arrow, before it detonates, and then throw it back because he is a heroic spirit.
Shirou can dispel his own projections, that is right, but I do think if Lancelot ever took over one of them, Lancelot would become the owner of the projection, and thus Shirou would not be able to dispel it anymore. Also, I am willing to bet, that with some good luck rolls, Lancelot could catch a BP arrow, before it detonates, and then throw it back because he is a heroic spirit.