I feel like all those "Diina = bad" are falling for a red herring. She obviously has her own agenda but she is also not a hostile (according to her at least) for real. From what I get from WNch40, I would say she wants MC to unite the world.
Also WN (Ch45) has a bit more on Demon King and Diina scene for this chapter.
Edit: Speaking of WN, the fan translation is finally ahead of the manga if people want to read.