RVR, the unfortunate accident with the Airtours crew was, as I understand, as a result of an engine failure, and the wrong engine being identified and shut down. This has nothing to do with single crew, as the same basic error at Kegworth occurred in a 737-400.
By your rationale, a 747 classic, is safer than a 747-400 because you have more crew members in the cockpit. The PPL is considered less safe, mainly because the aircraft only has 1 engine. This is not a debate about aircraft types, but the number of flight deck crew.
Light piston twins are simple aircraft, and therefore the pilot doesn't have the extra workload that a jet pilot has in managing the various systems....hence the requirement of 2 crew. Notwithstanding the fact that they go a lot faster, and are a lot heavier.
Lets compare apples with apples.