how is it that the curse weakens whether emma is alive или dead?

The яблоко оборот wasn't meant to kill her, it was meant to make her sleep. If she's asleep this would prevent her from undoing the curse anymore than she has. Because it's not so much Emma's presence that weakens the curse, it's еще her influence on the town and the people in it.

I think Regina is getting desperate and not really thinking straight. At least, that's my guess.