Try to keep it simple, and think of other examples:
1. "If I were you (but I am not) I would be very careful."
2. "If I were alive (but I am a ghost ) I would be able to eat."
So your alternative is grammatically correct in itself:
3. "If she were alive (but she is not) she would be 80."
However the example is something else. The missing part, that we have to understand, is in a different tense:
4. "If she had lived until now (but she did not)", or "If she had not died (but she did)" is followed naturally by "she would have been 80 on Sunday."
The different tenses reflect a different situation. In 1, 2 & 3 we are talking about the 'here & now' - a present situation (so I underlined the verb to show this). In 4, the defining verb that I've underlined is in the past.
So, today, if I were you I would be careful. But yesterday, if I had been you I would have been careful.
Have I made it better or worse?
Student or Learner