Airtags,
Whilst most of what you have said is completely valid, the issue of 'diligence' or otherwise of the cabin crew is a tricky one.
EG if a pax says, "I have a sore throat, is that declarable?", the strict answer is NO. The list of reportable symptoms are quite specific in the documentation given to cabin crew, and are reasonably obvious and extreme. However at the end of the day, it is up to the passenger to declare themselves(unless we notice something obvious) and if they don't....not much we can do I'm afraid.
That is not to make excuses for the example you provided, and yes that sounded slack if it is indeed true.
However, eveyone needs to remember, we are not doctors, and cannot make a diagnosis. Some pax seem to think we have miraculously had medical training and can now diagnose swine flu on a whim!
This whole thing is very messy to administrate, so we do our best...