Oakape,
I agree with much of what you have written immediately above. I disagree to some extent with your characterising of the Captain as just disregarding the published level.
I don't know who was flying or the circumstances. Just like you described briefing the wrong taxi path because of a false mind set which you picked up - not your co-pilot, it is entirely possible that the crew missed the discrepancy because of whatever their mindset at the time, and when it became obvious as the aircraft was level at 2500', the PF without realising the error not identified back in the cruise, made an assumption that they were cleared and down they went. After all s/he probably had a V-Nav indicator saying go down, and the MCDU indicated that they should be lower.
IMHO it is the systemic problems that lead to this, and the subsequent event, that need investigation rather than simply blaming the last piece of holey cheese. I say this in the knowledge that it could also have been a simple cockup.
I witness on a routine basis, people briefing approaches as they were taught in the sim. Sadly this method leads to the FMS/MCDU coding not being properly checked. Why were they taught this in the sim? Simple. It saved time given the inordinate number of approaches flown during an endorsement course, and reduced somewhat the time wasted on mouth magic. A negative aspect to this training, is that many seem to have lost the ability, or insight as to the need, to brief an approach in the sequence it will be flown, as opposed to just reading across the top of the Jeppesen/Airservices Approach Plate.