Reina, I suggest you just stay, the previous owner would prefer that, considering that he is powerless unlike his childhood friend. Putting him back will just lead to the same conclusion of mediocrity and inferiority. At least help your current version stand as an equal or surpass her, throw him a bone.
Anyone else suspect that the Reima of this world crashed out from an inferiority complex and intentionally summoned the soul of a stronger version of himself into his own body?
Since some of the dungeons have identical laws to his previous world, imagine one of them is an actual portal to it, and he gets to travel to the moment before his beheading and prevent it, allowing him swap bodies back.