As for why opposite sex shielding was harder, I always saw it as a function of saidin and saidar repelling each other.

When you place a saidin shield on a woman, apart from the strength issue, the inherent repulsion of saidar to saidin makes it harder to place the saidin shiled in the path of flowing saidar.

Same is true in reverse.

I find your own explanation problematic. You're saying that the gentler flow of saidar makes it easier to dam up. So you're saying you need a flexible shield. But we know that saidar shields are generally not flexible. Elayne in tDR compares the BA shield on them to a wall. And Rand does the same to the AS shiled on him. Berowyn's is the only flexible shield we know of. Talaan's shield on Nyn is also not flexible at all.

There seems to be no evidence for your theory, or the hierarchy you put up.