1. You have to go to the abyss and improve the facilities with renown for that
As for the spoiled part, kinda yes, but thats not new information, that was revealed on Silver Snow iirc
Sitri was created by Rhea but she ended up not inheriting Sothis consiousness. That wasnt the first time Rhea had tried reviving Sothis using that method either (not refering to the chalice, as thats a completly different ritual). It just happens that by coincidence this time it led to Byleth
Oh ok, didn't play SS so that explains why I didn't know that. Do you actually learn Sitri's name in SS as well? Cause that was new to me as well.