Linking DOES add strengths into one! Why else would they do it other than to add their strength to that of the one focusing the link? As for the issue of any 13 being able to take on any man, regardless of strength, I would regard that as a rule operating on the same basis that prevents men from linking or a 14th woman from joining a circle without men. It could be as simple as no matter what the relative strengths, some property of a circle of 13 makes it able to weave a shield on ANY channeler, no matter how much of the Power he holds.

The latter. Aiel are blindly and stubbornly insular to a ridiculous degree.

She's a coward and terrified of what would happen if she failed. Also, recall that Nynaeve beat Moghedian without channeling in the dream once. Why would a woman renowned for her cowardice try the same thing again, with a woman that much more confident in what she could do? And if the a'dam is a constant feature of her life, it would be more than a little tough to imagine it away in the dream.