It's "I'll be damned!" actually. you aren't becoming a "damn", you are undergoing the action of being "damned".
Sure. PH p.278, section on Critical Hit Damage. Note that "Rather than roll damage,
determine the maximum damage you can roll with your attack." So...when you have combat advantage, what is the maximum damage your rogue can roll?
The next bullet point tells you what to do with magic weapons and implements, and high crit weapons => you have to roll that damage separately. This makes sense: you do NOT normally get High Crit damage...unless you roll a critical.