Manga is finished but I'm just gonna throw my opinion.
The shrine goddess knows all the 6 girls will die, she could see their future, and MC fate is bound to all 6, not just his fated one.
So he have to juggle saving all 6, this theory is a bit weak, but explains how he died exactly liek the novelist was hinted to go.
Is super clear his childhood friend is the real or main fated one.
Anyway the goddess connected all 6 because she wants to relieve the boredom by making a normal kid do the impossible challenge to save 6 people, 5 that he doesn't know, and maybe grant him a violent harem in case he achieves it.
Childhood friend must have used the old tactic to provoke jealousy or force a loser confession, but failed for MC being more dense that a black hole core.
Story is good, but have some holes, the biggest one for now remains MC going for the selfie girl after getting his neck snapped when clearly seeing her walk away in front of his eyes.
And he is deliberately ignoring how the deaths changed when he interacted with his childhood friend.