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No. Becoming a homunculus doesn't mean immortality. But all Ling saw was the homunculi's regenerative powers, and the fact that there was no actual flow of qi within them. He's no alchemist, so he couldn't have known about what being a homunculus really meant. No one really knew, actually.
And Ed and Al couldn't bring back their mother because human transmutation is impossible. Once a person dies, you can't bring them back. Death is permanent.
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