Ruriha

That's not it. As to what Manaka truly is, your clue is already in the chapter. Again, as for the reason why Ritsuka is in Manaka's body can't be revealed. But it has something to do with her (Beast 0/L's) plan to recreate the world where it's possible for her to understand emotions.

I intended it to be confusing, lol. And yes, Manaka is a reincarnated individual, but there's more to it than that. And no, I will not say why Ritsuka is in Manaka's body. The reason will spoil this fic's ending.

No one from re:zero ends up joining the chat group. In one of the successful time loops, albeit where Manaka Emiya's name was eaten, they invited Subaru, but the Witch of Envy felt Manaka's true nature. And so, she killed him in a bid to reverse causality and overturn his decision from joining. The rest of what happened is what you can probably expect. What happens next would have been told in the next chapter of A Multiversal Anime Chat Group before I decided a rewrite.

Genderbend tag is for Ritsuka. He will find himself in the body of Manaka while everyone will forget what happened in this Singularity, but that's all I would say. Any more than this and I'll end up spoiling the ending.

Rancor means bitterness, the opposite of longing. For your second question, remember Lev? But if you haven't played F/GO already, he betrayed Chaldea at the end of Fuyuki Singularity. The same thing happens here, really. As to the reason why? Manaka wants to recreate the world where it's possible for her to understand emotions, pretty much.

Actually, nevermind. I forgot to add some stuff in there

While everything will naturally unfold in future chapters, the basic rundown is already in the chapter Timeline >.>

Tomorrow most likely. I'm already halfway done

This is the "rewrite", so no, I won't be continuing it. At least not anytime soon

Yes, but the members are currently hostile to Manaka, so she's going to avoid them at the beginning.