Escape path, altimeter error is not limited to forgetting to set QNH on transition. They could have been passed/misheard an incorrect QNH and set 1029 instead of 1019, for example. I've had that happen a handful of times, and was fortunate to remember the pressure in the weather brief or have got a Volmet or ATIS that said something different, catching the mistake. Perhaps this crew didn't have the same luxury of a cross check. It fits with why they went below real altitude minima heads-in, why the ADSB shows no major deviation, and fits with the most common cause for such incidents. It's all circumstantial, but don't shoot the idea down without evidence to the contrary.