A fine question, Lord Minkie...although it should probably be in its own thread.
Here's the answer (in two parts):
1) If you Chaos Lord is caught in the open, by himself, without cover and is hit by a lascannon -- he deserves to die. Never leave your valuable Lord open to that kind of damage...stick him in a squad!
2) Yes, IF your Lord is slammed in the chest by a powerfist or nailed in the face by a meltablast, he is TOAST. The Mark of Nurgle is an add on, like a Space Marine Bike, and therefore does not count. Anything that is Strength 8 or higher will blow him to pieces. In the same vein, a character with the Mark of Khorne and a powerfist will only have Strength 9, not 10, because the bonus is added after the powerfist increase.