Is it possible Asriel actually has his emotions and body back indefinitely? Asriel/Flowey is a habitual prankster and liar, even a little in the recordings. He says he'll become a flower in a little while, but the player can hike back across the whole underground and find him as Asriel again, where he doesn't mention it. His full power could have been enough to restore him. With all he's experienced, and his memories of doing terrible things, he might think he shouldn't go to the surface anyway.
To clarify, Flowey does have emotions -- what he lacks is compassion and the ability to love. However, after releasing the souls, Asriel is unable to keep his body indefinitely. The evidence is in who greets the player when the game is rebooted after the pacifist route’s ending.
If Asriel’s true form was restored, he shouldn’t be Flowey anymore. Furthermore, it’s not just a matter of Flowey atoning for his bad deeds that he chooses to stay in the underground. Flowey knows that appearing before his parents as Asriel would not help them move on. His time as Asriel is temporary, and he wants them to move forward.
» read more: the moral of undertale - part 1: letting go