Once I've been spotted by The Alien, is my death inevitable?
In my experience, most of the time this is true.
However... there is one item that you don't have yet that can be used to drive the Alien back:
The Flamethrower
Unfortunately, its not infinite so once you have it you still have to use it carefully. Also, when the Alien is scared off, if you're between it and the closest ceiling hatch, it WILL knock you over when it flees, dealing a fair amount of damage.