As for the latter, since you brought it up:
The thing about stuff like "How many Zebras are worth a headache." is that if you think about it enough, you end up at questions like "How many zebras are worth a human?" and then "Are all human lives equal?". When trying to answer such questions things get fuzzy and icky and hard to answer satisfactorily. Then theres versions of those questions where you ask if the answer changes if it's your life being weighed.
Even though it's intuitively obvious that somebodies fleeting minor pain is not worth the cost of losing an entire species of large mammal, talking about "fair trades" with sentient creatures gets weird and frustrating very quickly.[0]
[0]: I personally like the phrase "Infinite Hair" to describe situations like this. (ftp://ftp.trailing-edge.com/pub/rsx11freewarev2/rsx81b/374001/jargon.txt)