Responses
I also wondered about the second part. Did Rosamel mean that this is the only way HE can send them back, or is it the only way period? If it is the only way, then they were handed a death sentence the moment they were summoned. Giving them hope that they can be sent back is cruel.
When their soul is separated from their body they return to their original world which means death. According to Rosamel. Then will they be just wandering spirits without a body when they return to their original world? If so thats cruel. And the priestess had told smirkingly "we ll send them back when they defeat the demon lord" which means they will murder the heroes summoned from another world to get rid of them! This is demonic.