I've done job interviews online and they weren't too bad. The problem with these video essays (?) is the huge range of questions. If you can get a sample of the kind of questions they might ask then you can be better prepared. Maybe.

I agree. By now, we should all have a story about how we screwed up or made mistaken assumptions or took some sudden initiative and succeeded beyond our expectations, and ended up the wiser for it. Maybe this is the time for that story. Without a word limit you could go in to some detail.

400 words? I don't think so. 700 is reasonable but it could certainly be a bit longer. As much as everyone likes brevity, I think the HBS adcoms will be prepared for something longer than the short essay format. Just make sure it's interesting!