Maybe could have / should have done better. However, I believe the manoeuvre would have been completed successfully if the engine had reached and maintained full power.
For me, this is the primary cause.
At 100kts, upside down, the aircraft have to go into a vertical dive to gain flying speed, or enter a spin with injudicious control input.
'Primary cause', absolutely not. If he initiated the pull-up 50ft higher the manoeuvre would probably have been successful. But again pull-up height was not the root cause. Continuing to fly the loop having failed to achieve the correct gate parameters was the root cause. If the engine was misbehaving and caused a lack of performance then the pilot is solely responsible for noticing the problem and then taking a suitable course of action.
As highlighted in previous posts, the actual apex of the loop is not the best place to try to spot roll out if it. No, neither do you wait for the vertical when it's too late. Ease off on the pull, let the nose continue to drop below the horizon, IAS rapidly increasing, aileron roll 180 then ease out. You could probably achieve the entire recovery within 1000ft or less.