For Edea, think about it this way. If you met an exact copy of your mom who treated you just as your mom did, but your mom that actual raised you was still alive and existed, would you leave the mom who raised you for the one who is an exact copy and raised a dead you?
I hope that makes sense. Basically i can see where Edea is coming from. Her real parents, the ones who actually raised her, are still alive, and regardless of how identical they are, you were only raised by one of them. However for her parents in the world where she died, that does kinda suck for them... But I imagine its not as horrible as you make it sound. I imagine it more like "Hey, our daughter died, but she lives on in another world", IDK in some JRPG logic it might give her parents comfort to know their daughter still exists outside of this world...
Anywho, Agnes reason to go back might have been because the original world needs a Vestal, but i mean, the storyline basically says the world doesn't need a Vesta, no? Anywho, Tiz still made a retarded choice no matter how you look at it. Agnes had a kinda reason, Ringabel would probably want to be with Edea, who was dead in the last world, and Edea likely wanted to be with her real parents, not alternateworld ones. Tiz just.. IDK, he had /nothing/ left for him in the old world. Dead parents, Dead brother, dead everything... all he had left was Edea/Ring/Agnes, so if his only conceivable reason to go back would be for them.