Can I use a PC to control fans connected to the NV-FH2 when using its barrel power input?

Yes, this is possible, but some precautions are required.

When using a 12V power supply, please either make sure that the power supply is shut off whenever the PC is shut off or modify the NA-EC1 input cable by cutting the yellow pin of the cable to make sure that no power will go into the PC when it is shut off. If using a 24V or 5V power supply, the power pin of the NA-EC1 cable must always be cut.