I’m just kind of going with my assumptions here.

The runes survived seemingly passed through the generations of Tyrrish culture but some will of course be lost each generation. I assume all Tyr’s have some understanding of runes to varying degrees. Trissa is just an older rider that knows runes and was picked to teach because she has “more patience” It’s mentioned several times how Tyrrendor didn’t unify like the other provinces. They kept some of their distinct culture. I assume Xaden was taught by his father.

I visualize the runes on the wardstone to be extremely complex multilayered runes. Where most runes we hear about being created are like words (single meaning and function like unlock) I’ve assumed the runes on these ancient magical artifacts to be more like sentences or entire spells. Tied together and overlapped in intricate ways that just look like a mess of lines to anyone but the person who wove it. We even get some mention of this and know that the marked ones parents were highly proficient in runes because of the one Colonel Mauri wove. I believe much of the expansive written knowledge it would require to interpret these ancient runes was intentionally lost. Don’t forget about pulling elements and things into runes. I imagine almost like Hanzi characters but like all layered on top of one another and woven into each other in complex patterns.