Yep.

So basically, whatever light it is that those darkfriends saw, it was something more "spiritual" that they saw/experienced even through solid walls.

I think the madness is definitely there, but the impression I got atleast from Nynaeves description was that the light was protecting Rand from the madness. Look at the way it is described that the in each place that the taints thorns go into Rand's brain, the light goes there too, coating the thorns.

Also, lets compare Naeff and Rand. Naeff is functionally insane, but sees Fades all around him and is convinced they are true, and cannot see that he is mad even if its pointed out to him. When Nynaeve heals him, she finds a spot of darkness and discovers the thorns. Now it is not made certain how many thorns there were in Naeff, but based on the description of how Nynaeve has to use weaves to keep the thorns from sticking back, I would say there were maybe a couple of dozen of them in Naeff.

In Rand however, the darkness of the taint covers his ENTIRE mind, and there are apparently several THOUSANDS of the dark thorns sticking into Rand's brain to the point that his brain is a virtual pincushion at this point. Now it does make sense that Rand would be so badly tainted ofcourse. Rand has channeled MUCH more of the tainted OP than Naeff thanks to his repeated uses of the angreal, Callandor and especially Choedan Kal (twice). Heck, during the cleansing he practically channeled all of the taint through himself into Shadar Logoth.

The problem is, if a small patch of the darkness and a couple dozen thorns in Naeff was enough that he was showing clear signs of madness, how is it possible that Rand, with thousands upon thousands of them, was able to maintain even the slightest control? With that much darkness in him, he should not have been functionally insane like he has been. He should have been utterly and completely insane with no understanding of reality at all. With that much taint, Rand post-cleansing should have been convinced he was actually a duck and been running around flapping his hands making clucking noises. And yet, he was not. Heck, as Verin says, most of his madness is more psychological due to immense pressure brought on by his status.

That is why I speculate that the light has always been there, shielding and protecting Rand. The only reason we now see it is because Nynaeve was not skilled enough in delving to detect it before now.

I figure Rand's usage of TP and the darkness he sank into in his attempts to become Cuendillar caused this light to grow weaker, allowing more and more of the taint to reach Rand. Moridin and the Dark One could be aware of this, which is the reason Moridin is giving orders to cause Rand mental pain, because he knows that the worse Rand gets, the weaker the light in Rand's brain gets, and the more the DO's taint is able to influence him.

Moridin and Rand are linked, and will eventually merge. I figure that the more the taint reaches Rand, the weaker his side of the link becomes, and the more control Moridin is able to exert on Rand. We saw this especially in tGS when the more Rand fell into madness and darkness, the more he seemed to mirror Moridin in his philosophy about how it is all meaningless. The sickness in Rand also become worse and worse the darker Rand become, another sign perhaps that he was losing the merging-contest.

Now, after Rand had his epiphany however, the light in Rand's brain has received a massive boost in power, it is once more able to counter all of the taint and cover all of the thorns, and this is why Rand has regained his full sanity now. The sickness he used to suffer from channeling is also gone according to Min. The light-shield in his mind is apparently able to protect Rand from that effect of the Moridin-link too, now that the shield is fully charged.

That is my idea anyway.