"何を言っている __のか__ 、分からない" vs. "何を言っている __なのか__ 、分からない"

Question

In the recent thread titled Reason for 「のか」 and 「かもしれません」 in 「何を言っているのかわからないかもしれませんけど」, my impression is that saying "何を言っている のか 、分からない" sounds abrupt (and sort of impolite). While I have no idea if this is correct Japanese, I would have said "何を言っている なのか 、分からない". The "な" seems to take the edge out of the straight-up "のか".

The answer to the thread titled 「なん」as a formal, spoken, suffix for questions? mentions the phrase "なのか". But, I can't figure-out if that answer is applicable to my desire to say "何を言っている なのか 、~" instead of just "何を言っている のか 、~". I also found this thread titled What exactly is なの(nano)?. That thread looks relevant to my questions in this thread, did it really only confuses me.

1. Is it even possible to say "何を言っている なのか 、分からない"?
2. Would saying "なのか" make the sentence less formal?
3. Would saying "なのか" make the sentence less aggressive / abrupt? more polite?
4. Would saying "なのか" make the sentence sound more feminine?
5. Would a native speaker actually ever say "なのか" in this type of sentence in a conversation?

Answer 1

As I wrote in reply to kingyo's question:

　どこだ　＋　のだ　＝　どこなのだ

That is, な is the 連体形 of だ used before the formal noun / nominalizer の.

In this case, だ is simply not possible:

  *何を言っているだ

Generally speaking, you can't follow a verb with だ like this. And so the longer sentence:

  *何を言っているなのか分からない

is ungrammatical for the same reason. Since it's ungrammatical, I don't think any of your other questions are relevant.