One of the comments on the video suggested it sounded like a work by a Akiko Shikata - a composer I was not previously aware of in any way.Someone (Arlinn) pointed this out on the main site chat
https://www.youtube.com/watch?v=L9JNH8p26wo
This vocalist sorta sounds like Yuriko but the singing language isnt Kajiurago, its japanese mixed with something else.
No staff credits apart from 3 and 3 VAs on the website yet which is weird https://silentwitch.net/ there seems to be one more PV (1.3) coming out at some point). The bgm behind isnt Kajiura for sure.
It's 100% not Shikata singing (I would know) and I don't think it's her composing (and I doubt she would skip on singing it herself if she were composing).One of the comments on the video suggested it sounded like a work by a Akiko Shikata - a composer I was not previously aware of in any way.