Very good theories from the OP. I find it very difficult to believe that someone was so stupid that just accidentally set a huge sell order without price limit..
It definitely happens from time to time, you would think someone having 5000coins will never make the mistake but the truth it can happen, it is less likely than someone making a 1BTC mistake but it can happen in a sell off
I don't think we will ever know for sure what happen but we cannot rule out the mistake, it is still the most probable explanation even if it seems weird