Lots of talk here about aircraft design, maintenance procedures and so on.
If the final report shows that the engines were left un latched, then it boils down to an inexcusable act by both the engineering team AND the pilots.
Aviation learns from its mistakes, thus why we have second engineers checking and counter signing work once complete. The flight crew should also have noticed this on their walk around. It is a known area of concern, not just on this aircraft/engine, but any where the cowl latches are of a 'stealthy' design. If they didn't look, they didn't see it, and thus inexcusable.
Not to take away from the skills demonstrated later, just to to say this could likely have been avoided all together.