Yeah, I think there were many ways to bring Prue back or least some! Grams said in the first eppy of season 4 that Piper can´t talk to Prue YET!!! But later she could come back as ghost least sometimes to help her sisters!
For Leo it was too late to heal her? Remember the episode in which Piper had this fever. Even as she was a ghost in a kind of room he brought her back to live although it was forbitten! ...
Poor Prue! I miss her!