A2 is co-located with B1 at a single event. It happens just once. Comparing the time of that event with events at different places depends on your reference frame, because simultaneity is relative. But in any reference frame, the events happens only once. No. In every reference frame, the event of A1 meets B2 happens before the event of A1 meets C2. "When A1 is in transit from B2 to C2" isn't well defined at B1, because simultaneity is relative. How does B1 decide when A1 is in transit from B2 to C2?