I think it's pretty well clarified by the end of AMoL that the process of "turning to the shadow" is actually removing the influence of the creator from the person (similar to how in Rand's 'perfect' world he removed the influence of the dark one). Without that influence, all of the person's other attributes are still the same, they just no longer have any desire to be or do good.

The reason that people who were strong *for* the light are then strong against it is because those were the people with real strength of character, determination, etc. After being "turned" all of that still remains, just lacking the impetus to strive for the light.

Basically we have a statement that says "Those who were strongest for the light became the strongest for the dark after being turned".  That A implies B does not always mean that the inverse of A implies the inverse of B; thus, it's not necessarily valid to say that "Those who were not strong for the light would not be strong for the dark".

I don't think Moriden was ever Turned; I think he did it of his own choice.  It is pretty clear that those who had to be Turned generally are less effective.  In the world RJ created you have to have some influence both ways to be complete; that's why the dark one couldn't be destroyed.  I think that the ones forcibly Turned were less capable because they lacked that impetus to the light just like the ones in Rand's 'perfect' world were not themselves without any impetus toward evil.

While I certainly won't argue that Thom had a major influence on keeping things whole, Brandon Sanderson actually stated (in a tweet that I can't find right now) that the thing that Moiraine had to do to save the world was to make sure that Rand and Egwene arrived at the last battle together and not seperately.

In addition, while Egwene wasn't actually the one to break the seals, it's entirely possible (probably likely) that if Rand hadn't given them to Egwene they never would have been stolen by Taim's lackies (they were taken from Egwene; Rand would have known immediately if he'd still had them) and Logain wouldn't have ended up with them.  Hard to say what would have happened at that point.