Going through the points;
Coup-de-grace
Coup-de-grace (about 2/3 down, sadly no direct link) does not specify if the victim makes any noise if they fail their Fortitude save of 10+damage dealt (and thus die) however if they make the save then they certainly would wake up.
This then becomes a DM decision. I personally would make a second Fortitude save for the monster against the same DC with a modifier of the characters stealth bonus rather than the +10, which takes into account how good the character is. If this is failed then the victim expires noisily, this then gives others nearby a chance to hear/wake up.
Waking giants
The DC to use perception when asleep is +10, meaning a base DC of 20 to wake if they are asleep, modified - as you say - by noise of the weapon, once battle starts this is -10, which means a DC 10 to wake up.
Note: There is a lot of debate about battle at waking up on the pazio forums
If the giants wake up then they will be effectively flat footed and surprised for a round, the round thereafter they would roll initiative as normal and remain flatfooted until their go.
Stealing from giants
Stealing a small item from a sleeping giants body will be sleight of hand DC 20, the giant gets a penalty to detect this as they're asleep (+10). Stealing larger objects should, as you say, give a bigger penalty (although not covered directly by the rules). If the bag of loot is merely beside the giant then stealth should be used instead and the giant again makes a perception roll vs. this.
The standard coin weighs about a third of an ounce (50 to the pound).
Sizes and weights of monsters are listed here which gives you the weight of an person for small of "8-60 lbs" quite a lot really! But there's no weight modifier for objects as these are related to the size of the creature that is using it - ie a large sized dagger or small sized dagger for the creature wielding. Just a weight modifier for armour themselve. (about three quarters down, "armour for unusual sizes")
This means it's down to your call; I'd scale it up with multiple -2 penalties for every 10 lbs they're trying to lift, which is 500 coins as they're more likely to clink clink clink.