Nope, SpaceX edited the video. The simplest way to tell this is because the last 2 or 3 seconds show fairing separation, not booster sep.It's more likely that the down stream terminated upon destruction of the vehicle. Rather than SpaceX actually editing the video.
Surely. SURELY they tested the new first stage motor, and noticed residual burning after cut off. I'm no part of SpaceX, and don't claim to know anything more than their engineers do, but it seems that this issue should have been observed and fixed on the ground way before the launch.
I mean, any ground fire test of the new first stage engine would have shown residual burning, right? How this couldn't have been accounted for in the staging timing is beyond me.
Yes, the residual burn was visible in the ground test. There was a video of one of their tests published a while back, and that clearly showed this burning. A couple of my colleagues (who know more about engines than I) even suggested it'd be a problem, as did many others. None of whom worked at SpaceX, however.