Well, literally any written symbol meant to signify a concept or convey an idea counts as written word so clearly they don't feel as strongly about that if they have licenses.

Qestir likely means silence or something along those lines in Auri Language so that's where they got their clan name probs.
The issue of given names is certainly a bit hard to figure out. It could be that they pick their own names by just like... pointing at a thing and then themselves? I dunno if I was a voluntary mute that's what I'd do. Could be they're written down. Another possibility is they're given names by that bloke who speaks for them in Renunion or some similar figure.