Medieval orthography

[Exez]

The article on the Verdurian alphabet says that the letters for sh and ch were derived from a more ancient Cuezi letter.

Before this introduction, for this sound a ligature of s+c was used.

Do we know the form of this ligature? did it evolve in some other letter in the other scripts?

[tezcatlip0ca]

This, maybe?

[Exez]

No, shes is a reversed chen, which itself is a borrowing from Cuezi yetu

These replaced the ligature I am referring to (or so I know)

[tezcatlip0ca]

When a y seemed to be needed; it was taken from Cuêzi; other writers, however, had resurrected the same letter (or perhaps developed it from s + c) to represent both š and č. Eventually this confusion was sorted out by modifying y in the direction of i to create a new letter i brevë for y. This freed up the chen to represent č; this letter in turn was flipped for š, and from this the creation of ež for ž (and j in languages which needed it) followed automatically.

you forgot this part, Exez

[Exez]

So shes/chen has two possible origins?

I thought that "or perhaps" means that two options (yetu and the s+c ligature) coexisted for some time, until yetu dominated and replaced the ligature