The eavesdropping trick was revealed later. I think you are underestimating the mindsets of the Aes Sedai by over-generalizing across the entire population. Further still you are minimizing the aspects of Moiraine's channeling that step outside the box of what the other Aes Sedai can do - like it or not, using a staff, wand or other object as a foci to channel in a certain way is creative, even if you personally see it as a handicap. These aren't even treated as limitations the way learning to weave something one way could be seen as - she can still throw fireballs the Aes Sedai way, but she has other ways, as well. We don't see other Sisters use her trick with the coins (the one used to track the boys in TGH), for example, and for all intents and purposes, this might be another weave she created on her own. There is no reason to believe that the personal tricks Aes Sedai keep to themselves are limited to one or two Wilder skills, and I'm inclined to believe that the range of 'secret' weaves a Sister might have could have occurred through many ways - perhaps a Novice did something unexpected there and combined with what the Aes Sedai has seen elsewhere, she figured out how to create a particular effect (this is exactly what Verin and Liandrin say in separate POVs later one). Even if the secret is a difference in technique and variation, it still implies creativity.

Finally, if she didn't learn it from another Aes Sedai, it's a difficult weave that only a wilder of Nynaeve's strength figured out in a moment of wild (plot-driven) channeling, and a weave can't be written down given the complex nature of matrixes (and if they could be written down or portrayed, there are multiple other weave, that would be more useful to preserve to the Aes Sedai than one of the weaves that they forbade themselves to use since the War of Power) then how else could she have learned it? I would say the simplest answer is "The same way Verin or Liandrin learned Compulsion and the way Egwene figured out Skimming...she figured it out on her own through experimentation and guess work."