Great discussion. In fact, so great I registered for an account and this is my first post.
I am guessing the rules governing dahn or nahn are like the rules that have govern heretical stations throughout history: rules are rules unless you have a lot of money or your own army in which case rules are suggestions.
One potential theory would be that all levels of dahn or nahn are stable but position can increase the rank. With this idea, Shallan's father is fifth dahn like her but being a lord increases his dahn by one. All of his children are fifth dahn but upon his death, Balat should have moved to fourth dahn ("should have," because they kept his death a secret). Renarin is 2nd dahn, but had Sadeas plan in WoK worked, he would have moved to 1st dahn.
Tenner is reserved for those whose eyes are light but 1) their parents were tenner, 2) they were stripped of rank (like Dalinar wanted to do Roshone), or 3) they are disinherited (Balat predicted he would be tenner when he fled the estate).
Not sure I believe the theory but thought I would throw it out.