[Kiwami]

It's basically Shiro & Yui who purge Mirror World from existence by going back in time to help their younger self’s overcome the depression. Which results in the disappearance of Mirror World form the time line, so yes: Nothing Rider-related ever happened.

Ren basically just fulfilled his wish to be reunited with Eri.

And the Megumi thing.. well I just take it that some things are destined to happen. Like Megumi will still be fired by Kitaoka and end up at Ore Journal.

I wouldn’t say all the development for the characters vanished... they just learned it another way.

It wasn’t my ending of choice either. I thought the best way to handle this was that Ryuki won the war and his wish would've been to go back in time so he can rescue Shiro & Yui before their parents go completely crazy. He could also make subtle changes to the time line, like bring Kitaoka to a doctor so his sickness can be cured before it gets serious.

But that's just me, I can live with the ending as is.

There's an alternative ending also, maybe you should check that out.