Yes, you can use it for the SMN avatar battles. Plan carefully though. You've already drawn hate, rememeber.
Yes, any job can use it. Any level can use it.
Yes, there is a time limit. Use the scroll when you enter a dangerous zone, perhaps...if you know you won't be in it for hours. I'm not sure what it is...perhaps 3 hours? Can anyone confirm?
When you die with this active, do NOT choose to go back to HP. Just wait until it asks you if you want to be raised. That's all. It doesn't happen right away either....so wait a few minutes, if needed. Lag, your comps speed, etc...all affect it.
And yes, you will have raise sickness. Despite what others have said. So make sure the mob(s) that killed you are out of the way.