"It was all a mess but descending to circa 400' while not even halfway down the downwind leg shows clearly that neither pilot had a clear picture in their mind. The simulation was just staggering to watch. "
Actually, they descended to just above 200 feet because, although not shown on the PFD, the flight director was on (FD1), and vertical mode was V/S -500 on the FMA. V/S mode happened after the ALT selector was placed to 3000 feet. The autopilot had kicked off for some reason, and the PF was distracted by the pitch command bar commanding a V/S of -500 FPM. It is evident that neither was really aware of the pitch of the aircraft, and was simply following at least the pitch FD bar. A principle that should be universal is that: if you don't intend to follow the flight director, switch it off!
Fatigue may have led to confusion over complex flight management systems. However, there doesn't seem to be the standard callouts of FMA changes, so situational awareness was probably lacking and cross-checking would not be evident by the PNF. No callouts and acknowledgements indicate lack of standards by the airline.