How is walking a straight line in any way an objective measure of alcohol induced impairment? It's just another bogus "standard" like lie detectors that they seem so fond of in the US. At least the great god science should have some objective evidence to back it up as opposed to anecdotes and self assessed gut feelings.
Flapping wings relies heavily on oxygen consumption which varies with atmospheric pressure so AMSL is a better general measure of where you'll find the birds than AGL, unless they're soaring.
MEA. The science says nothing of the sort. it's called acclimatisation - just ask any mountaineer or performance athlete. Such rules are by their nature there to endeavour to cover everyone. How would you measure your tolerance to altitude o any given day? Wait until you feel hypoxic? What about when you have a cold or minor lung infection?
As to alcohol, you might feel okay but have you objectively tested your cognitive ability or reactions? Again, what happens when you're tired or unwell?