I'm glad the "collect idol photos" joke is canon. I think it's pretty funny Komine got sent back in time for such a stupid reason instead of explicitly getting a re-do because he has regrets or whatever.
On the other hand, it might be it was just an excuse for the alien. After all, if the alien had asked for nothing in return, it would have been more suspicious. It was an alien, not a god, after all. There's no such thing as a free lunch. The alien might have had its own reasons beyond Komine's (meager) understanding, so it chose a task someone as stupid as Komine would understand. This way Komine will happily keep collecting idol photos, instead of wondering what's going to happen one day, living in uncertainty. Now he lives securely believing the alien wants nothing nothing but idol photos.
But then again, this is just a silly romcom.