If a fighter has any willpower at all (and he does) I see no reason why he cannot enhance it through concentration just as he might enhance his reflexes.
Indeed. It seems simpler for me to have it hidden (along with all the other monster math) and just present a table of various levels and monster damage for the different monster types (brute, elite, minion, etc).
You already have that; it's called murder (I believe you may have already mentioned this).
There's no reason to believe that 1st level wizards should be able to put alert guards to sleep with 100% (or any, for that matter) chance of success.
Given that we only know about 1st level pregenerated...