[Wolfdrop]

Since Ce Tydonn is now what used to be Cengemis before being conquered by the Tydonni tribes, is the "Ce" part of the name a shortened form of Cengemis and is a sort of double barrelled name?

Cengemis + Tydonni invasion = Ce Tydonn

The Tydonni are never referred to as the Ce Tydonni, and the remaining Cengemi and called as such.

It was just a thought I had when I was trying to work out how to pronounce Ce Tydonn ( Say, Seh, Suh, Cay, Cuh?)

Pretty neat piece of world building if it was intentional.

[H]

I think you are right, in the sense that the "Ce" comes from Cengemis.  I think it's place is sort of a "administrative" or "historical" artifact.

Not a great parallel but similar to how we call this the United States of America.  Yet no one would call themselves a "United States-er" or a "United States American," so they call themselves Tydonni, as the Ce speaks only to the nation itself, not it's people, per se.