Old PSU works in the system. CX430 doesn't. Old PSU still works in the system. The CX430 is probably defective.
The red motherboard LED is powered by the completely separate standby power supply in the PSU. Except for providing power to the startup circuits, it has little to do with the main power circuits.
The CPU cooler fan spinning up without the CPU power plug being connected tells you that the 12 volt circuits will produce a little power but not a lot.
The LAN card problems are a symptom, not a cause, of your boot problems.
The CPU power plug in the Corsair PSU's will be a "4+4" plug. If you are using a "6+2" plug, you have interchanged the CPU power plug with a PCIe plug. And you are shorting 12 volts to ground.
When you plug the CPU power connector in, the yellow wires should be facing the keyboard/mouse connector. If they are, the PSU is bad. If the black wires are facing them, the power plug is in backwards and you are shorting 12 volts to ground.