καί μιν φωνήσας ἔπεα πτερόεντα προσηύδα...

And he addressed him speaking winged words.

This from the Iliad, in Book 13 line 750 (and of course, elsewhere).

I am quite cheerful for two reasons.

1. Zmjezhd at Epea Pteroenta has kindly consulted the real experts, and produced a series of more detailed etymologies for tenebrae. Check it out.
   
2. My intuition was not too far off. The consensus that emerges on the evolution of the [n] in tenebrae seems highly speculative, and so I believe I was right to be a bit skeptical about the derivatives offered in L&S. Nevertheless, Chris's argument remains elegant.