Will you be able to solve this murder case puzzle?

Question

CASE

June 6, 1992
A family of five was murdered after coming home from a party of their relatives. The family of Lucian and Safire Frenier were found dead in different places. Grizzly, Glinerva and Galathea were the names of their children. Two of the children, Grizzly and Glinerva, were found dead just outside their home. They were seated on a bench like they were just sleeping. When the police took a closer look, the children were strangled, their neck tied with a super thin thread from stockings. The last child, Galathea, was found posed with her dolly next to her body in the sofa of their living room, murdered just like her siblings. The letters V and C were written on her doll's left and right hands.

The bodies of the parents were trickier to find, until one day there was a famous thread about people seeing graffiti on the walls and stone beside a tree; it said, "Who put bella in the wych elm". The police investigated these places and eventually, the bodies of the father and mother were found on the trees beside the graffiti. Taffetas of the same silk were found on their mouths. Their bodies were stuffed on the trees.

There are some writings on their taffetas.

On Safire's taffeta:

4.Bqoiwcftx ou bqjzuaw hnxt
5.Fschnpbsd ou ebwyaelx wygd
6.Wjsgkdk gi qweb jqtaap

On Lucian's taffeta:

4.Vteeo dkysy zjts ll pnojcn inajsekb
5.Otwrk jki thhrby ll sriqsni befuroybfbr
6.Xsn gdnk yrrnn tjbr jxkj ll wpxyrn yqoncgen

Suspects

Samuel - A coworker and bestfriend of Safire.
Eizyra - A close friend of Lucian in a bookclub they've joined since they were kids.
Kuwit - Owner of a thread and silk shop where Safire always bought.

Answer 1

I believe there is a case to be made that the murderer is

Eizyra

Because

The VC on the doll's hands stands for Vigenere Cipher. Using the key "Safire" on the cipher left on Safire's taffeta gives
4.Jqjafynts gd xyjumjs pnsl
5.Ookhihkol op wksgazdg sggy
6.Osookyc pe ywzt smbavh
and on Lucian's taffeta we get
4. Vteeo dkysy zjts ll pnojcn inajsekb
5. Wtrjt fsi ozqnjy gd bnqqnfr xmfpjxujfwj
6. Fsi ymjs ymjwj bjwj stsj gd flfymf hmwnxynj

Then

Using a +5 Caesar shift on these, we get in order
ELEVATION BY STEPHEN KING
INSURGENT BY VERONICA ROTH
ZEITOUN BY DAVE EGGERS
YOURS UNTIL DAWN BY TERESA MEDIEROS
ROMEO AND JULIET BY WILLIAM SHAKESPEARE
AND THEN THERE WERE NONE BY AGATHA CHRISTIE
The first letters of each title reading down spell EIZRYA.

Additionally, the graffiti "Who put bella in the wych elm" is from a well-known historic case in England, and

is said to be the inspiration for the novel "The Wych Elm"

and

Eizyra is in the book club, and so would maybe have read this novel.

Answer 2

If this was meant to be red herring, well, I'll think again, but this is what I think:

It is Kuwit because he knows what silk they bought and he runs the silk shop. The children were strangled with a small thread, too. There are many other clues, but this seemed a little too obvious and now I'm doubting myself.