Cupidos

We know if it comes to worst Austin will not show mercy dude was doing his best for survival from the very beginning even if there is other goals this one is still the main one

Adding mutants will be not bad

İf Diablo betray I would just drop story for how dumb it become

Reinhard can see future so this war is over even before it started and Luminous is one of the more kind Demon Lord's but if she truly loved she would choose his side(this is canon Luminous) and all this part till emanation will be Reinhard flexing his powers

It doesn't make sense for Rimuru to possess Raphael without Reinhard losing it most probably both are his clone, fragments or descendants by Spector lineage either way if Rimuru was Fuji Ren his soul would not be similar to Golden Soul of Reinhard so it's seems not realistic but Rimuru and Yuuki can be Isaac and Johan

Clone vibe or power vise

I said that theory in last chapter's comment section

Where is the second chapter

Probably he and Yuuki are fragments of him or Spector Descendants of him without knowing in case of Rimuru

Then I hope you can expand from that part since it seems like you want to improve it