Agree with much of what you have to say,but essentially the lack of an effective walkround was a contributing factor rather than root cause.
Improvements in barrier defences help,but unless the roots of the issue are 'fixed' it will reoccurr and another aircraft will get airborne with an insecure fan cowl. Relying on human behaviour,in the case of the engineers & flight crew,is weak,the problem needs to be 'engineered out',as has already been aluded to.
The issue of engineers fatigue is a long standing one and i agree ,engineers can make a rod for their own back,by being OT grabbers! Unless their hours are regulated, in many companies they will continue to work excessive hours.
SMS for 145 maintenenace organisations may have an impact (if it is ever implemented) on FRM,but unless implicitly regulated fatigue will continue to have a major impact.