Theoryland of the Wheel of Time

My question is in regards to the writing system. In Warbreaker, when Siri is teaching Susebron to read, she mentions the letter "shash," which we now know better as a Glyphair from WoK.

so onto the questions:

Are the two writing systems related, or is this a chance coincidence of names? If they are related, did they stem from the same source? (i.e., do the people of Nalthis and Roshar both descend from a more ancient group of people?) If I haven't gotten a RAFO yet, did the separation from these other people create the legends of being cast out of the Tranquiline Halls?

There are interesting connections around the cosmere between linguistics and some cultures. Though different groups of humans were created on different planets, the Shards all share a single point of origin. However, the Tranquiline Halls legends are not related to a Nalthis/Roshar connection.