In the original RE2, the official canon is a mix of all four scenarios. But Claire A/Leon B is probably the base scenario, as there are certain things that are cemented in the canon that only occur in that scenario combination. This includes Leon starting out in the driver's seat side of the road in Scenario B (he was the one driving the police car), and Sherry getting infected by the G-Virus. Another thing to take note of is that RE2's scenario is basically a more fleshed-out and modified version of 1.5's. Conveniently enough, Elza and Leon's starting locations mirror those of Claire A/Leon B's. Elza starts in the precinct lobby while Leon starts on the helipad. Likewise, Claire enters the RPD via the lobby while Leon enters via the helipad area. Elza is also the one who meets Roy (basically 1.5's Marvin) in one of the office rooms, which again mirrors Claire meeting Marvin in Scenario A. Elza also arrives at the factory and lab before Leon does, which mirrors Claire A/Leon B's progression. If I had to guess, the reason why Claire A/Leon B was intended as a "second run" is precisely the same reason why the game had an A/B scenario for the characters: Leon A/Claire B is the "Scenario A" while Claire A/Leon B is "Scenario B". Scenario B being the one that gives you a complete picture of the events of the game.