Fortunately/unfortunately, whatever way you chose to look at it, all you had to do to get your name on the list was to send them an email saying you wished to attend. Anyone could have done that and when looking at the attendance figures you should keep that in mind. I could have used all my email addresses with a fictional name if I really wanted to!
There was no bond between the two sides. For example, when applying for other jobs you may have to supply copious amounts of information - maybe answer a few essay type questions - you'd want to attend an assessment after making such an effort! Clearly the majority of non-attendees forgot to let Etihad know which is a real shame.
There are numerous examples of disappointing turn out in life. Airlines over book flights for that exact reason. You don't invite 50 people to your party and expect a 100% attendance (unless you advertise it on myspace in which case you get 500% attendance) People book tables at restaurants and don't show up - c'est la vie.
With regards to your costing of assessment, that's the risk Etihad take when conducting a recruitment drive. You just don't know what will happen on the day. Good thing the snow showers over the UK didn't occur a few weeks later - that would have certainly killed off a few extra attendees!
NOTHING at a first rate airline matters as much as your seniority number/date of hire. It determines pay, position, schedule, vacation, time spent sitting reserve, and quality of life in general.
The airlines I have worked for have promoted on merit and not staff number and so they should too!