We're already seeing reports of the 12HPWR connector not being able to handle the power the 5090 demands [1]. It seems that for this generation, NVIDIA left little headroom. Might be best to run these undervolted to have some of that headroom back.
It's more that it's perfectly happy sending all 600w over a single wire if the resistance on the other wires is high enough. Since there's a single point of connection to the card and a single shunt, there's no way to do load balancing like when there used to be two or even three 8 pin cables.