I highly doubt that you have to find specific walls for words that are involved on seemingly random descriptions, let alone like that's the only method available to one in the first place. The question lies in how these bloody things can give of "whispers" in the first place. Perhaps any word written in Dovah-script has the potential to give one off.

Hence the 10% lee-way for possibility.

I don't think there is an established definitve answer on this. But I agree. I doubt the Greybeards actually traveled to even half the word walls if any at all. I think they are just convenient for Dragonborns or potential Thu'um using wannabes. As for the whispering? Well your guess is a s good is good as any. I think words of power can emanate to just about anyone. There are a few records of common mortals hearing "chanting"(one diary from Solsthiem - the barrow with the spiders if I remember correctly, it's been awhile). The Dragonborn can of course comprehend it and draw knowledge from it immediately.

I think the word walls are a convenient tool for the Dragonborn to take advantage of. They are by no means the only source to learn shouts.

However this still leaves doubt for Tiber knowing Dragon Aspect. It's pretty clear that Miraak created this shout. How could Tiber have known about it without having some contact with Herma Mora?

Tiber Septim didn't need any word walls. He didn't even need any words. Ever heard of Nonverbal Shouting? Of course you haven't. The Greybeards didn't trust you enough to tell you about that one, and Paarthurnax only taught Talos.

Source?

http://www.youtube.com/watch?v=eW4tY8mQ_1o.

Or more pertinently, every single source on the Thu'um before Skyrim does not mention specific words. It also mentions Daughters of Kyne teaching it to humans as opposed to Paarthurnax. I propose an alternate history, or at least a supplemental one - not all Shouts are in Dovah, and not all of them contain words - and even the ones that do can be done nonverbally. This prevents the previous lore from being completely retconned. Both stories can be true.

Here's a really good example involving a Glenmoril Witch:

Suddenly, something invisible swept through the bushes like a wind. The orcs flew backwards, falling dead on their backs. Turala turned and saw a wrinkled crone with bright red hair emerge from a nearby bush.

"I thought you were going to bring 'em right to me," she whispered, smiling. "Best come with me."

The three followed the old woman through a deep crevasse of bramble bushes that ran through the field toward the house on the hill. As they emerged on the other side, the woman turned to look at the orcs feasting on the remains of the horse, a blood-soaked orgy to the beat of multiple horns.

"That horse yours?" she asked. When Cassyr nodded, she laughed loudly. "That's rich meat, that is. Those monsters'll have bellyaches and flatulence in the morning. Serves 'em right."

"Shouldn't we keep moving?" whispered Turala, unnerved by the woman's laughter.

"They won't come up here," she grinned, looking at Bosriel who smiled back. "They're too afraid of us."

Turala turned to Cassyr, who shook his head. "Witches. Am I correct in assuming that this is Old Barbyn's Farm, the home of the Skeffington Coven?"

"You are, pet," the old woman giggled girlishly, pleased to be so infamous. "I am Mynista Skeffington."

"What did you do to those orcs?" asked Turala. "Back there in the thicket?"

"Spirit fist right side the head," Mynista said, continuing the climb up the hill. Ahead of them was the farmhouse grounds, a well, a chicken coop, a pond, women of all ages doing chores, the laughter of children at play. The old woman turned and saw that Turala did not understand. "Don't you have witches where you come from, child?"

"None that I know of," she said.

"There are all sorts of wielders of magic in Tamriel," she explained. "The Psijics study magic like its their painful duty. The battlemages in the army on the other end of the scale hurl spells like arrows. We witches commune and conjure and celebrate. To fell those orcs, I merely whispered to the spirits of the air, Amaro, Pina, Tallatha, the fingers of Kynareth, and the breath of the world, with whom I have an intimate acquaintance, to smack those bastards dead. You see, conjuration is not about might, or solving riddles, or agonizing over musty old scrolls. It's about fostering relations. Being friendly, you might say."

I'd say the Last Dragonborn is at least a match for Ysgramor. He/she's a Dragon of the North him/herself.

Fairly certain that isn't thooming. She even says conjuration.

That isn't the voice. In lore, the voice has always been referred to as the Thu'um. Skyrim comes along and fleshes out the idea - explaining exactly what it is. It seems odd that Skyrim's story arc would focus on not only the voice being spoken in Draconic, but even more odd if we are to believe Tiber didn't shout in dragon. The dude was summoned and received training from the Greybeards. And then shouts not in draconic? What?

This is a big assumption you are making here.

I'm not against creativity. But this just makes zero sense. It contradicts virtually everything Skyrim's lore added about the voice.

The "everything goes" mentality is just as annoying if not more so than the "that's stupid and you're stupid" mentality.