TD, you wrote:
It appears then that an "Aircraft Ground Engineer (AGE) of SNCO status" is the person who having checked the ac, signs to say that the ac is fit to fly.
That is some responsibility then isn't it .
Massive. But, he can only take responsibility for the actions that he signs for. He does not open up panels and doors to check fluid levels. Someone else has already done that (and certified that there is sufficient fluid). However, the AGE will certify that the panels and doors are shut before flight. Overall, systematically, his job is backed up by solid teamwork by those around him in the engineering world and the aircrew themselves.