[Autumn]

10% on hit, up to 40%, does the job just as well without entirely removing the weapon's threat and forces investment on the levels of (if not greater than) Intensify Cold, which has less counterplay than River Sword. How is that 'so convoluted'?

I think that comparing a 9m cast from 1 class to a 3m bop from 1 item is kind of a silly comparison, I would argue Intensify cold is extremely different all things considered.

I actually have a slightly different concern regarding making it 10% stacking, which is just that they might be dead before you get much benefit out of the weapon.

Seeing it that way, I can understand the concern as well, it'd take away most of the viability of the weapon and just leave it in the dirt, how about..

River Sword:
Duration and Elemental weakness remain the same (40% and 4 rounds)

Hitting an enemy with water damage will reduce the duration of the elemental weakness by 1 round per.

I think this would be worth testing, at the least.