1. You should not be able to sink an undamaged CV with a formation of Ju-88s. If you can, then there might be a bug since the F5B torpedo had a maximum 551 lbs. of explosive charge. That's only 6,612 lbs. total for a formation including the doubling of damage to take into account the effect of hitting below the water line.
2. It's extremely rare to be able to hit from directly astern. You cannot drop at the moment the fleet ack begins to chew you up and hit the CV. Axis torpedoes are fast, but have a short travel time. They cannot catch the fleet before fuel is exhausted. Slow torpedoes (US models) have no chance of overcoming the speed differential from astern, even though they run longer.