When ET and the flower are brought back to life, I always thought it was because his people zapped the general area with a reanimation ray. That’s why Eliot asked “Does this mean they’re coming?” (To which ET replied “Yes!”)
ET was dead as a doornail. How would he ever be able to resurrect himself? Despite the Jesus allegory, I always considered the movie from a scientific (not religious) viewpoint. Once you’re dead, you’re dead, unless there’s some kind of external intervention.
All of the above, but he needs symbiosis with his own people to maintain his life force; and if he is separated from them, he has a certain amount that he can use for himself and also on others, but they are a drain on him and he is only replenished when he is around his own kind.
Since the zombie has arisen already - years of sci-fi movies and books notwithstanding, the odds of the physiology of an ET matching close enough to anything on Earth to serve as a breeding ground for bacteria seems to this non-biologist to be extremely low (and let’s not even talk about half-Vulcans . . .)
Jumping into all of those pits looking for telephone parts is why E.T. got sick. If you’re going to be hanging around in a bunch of pits, you’re bound to come into contact with some really disgusting stuff. And the stress of constantly having to run away from all of those federal agents is bound to compromise your immune system. I hope they had some fancy antibiotics on the ship.