DOVES:
Regarding your point 2, no the flash at 4s is not an explosion. As someone pointed out already it is just a strobe light on the plane. You can see a previous flash at 2s into the video, reflected from the ground.
I'm sure someone might know the 737 strobe interval, but 2s is about right compared to this video:
Any explosion is not going to be so brief (single frame) even at the low frame rate of the video.