I think the second load didn't even explode... it just obstructed the barrel as it was a powder/ball/powder/ball kind of load...
Highly improbable...once the barrel began splitting there would not be anything to separate the powder in the 2nd load (if there was one, I doubt there was based on the amount of smoke). The burning gases from the charge in the rear would engulf the forward charge and it would burn as well. As I said, no evidence of a second charge going off after the first and not enough smoke.