So . . . interesting question about inalienable possession!

First of all, bravo on the statistical analysis. 😊 That's exactly the best way to find the data you need to form linguistically significant generalizations. (And thank you for reminding me what I said back in 2010!)

You're right: regarding the genitive/topical distinction in Na'vi, family members do not count as inalienable. So the expected form is your number (1): Oeyä sempul längu skxawng. (I hope that example was not inspired by real life!)

However, there are circumstances in which (2) is valid. Consider this little bit of conversation:

Oeyä sempul lu kanu sì tstunwi.
Oeri sempul längu skxawng.

Here, A is saying something about his/her father, and B responds with a contrast, as if to say, "In my own, situation, unfortunately . . . ", or "Unlike in your case, . . . " or "As for me, . . . " In other words, the topical is acting as a pivot, where the focus is changing from A's situation to B's. That's one of the useful things the topical can do in conversation.

If you wanted to communicate the same thing in English, the usual way would be with a change in emphasis and intonation:

My father is intelligent and kind.
MY father is an idiot.

Hope that's all clear.