READYNAS NV endless reboot - new PSU - Reset no luck.

From logs and symptoms, I am pretty sure that power button is the culprit, prove:
(1) When you plug in power, it power on by itself. This is not normal behavior, NV/NV+ should stay off until you press power button
(2) All restart recorded as cold startup, not warm reboot. If you pay close attention, you can probably note that there is a short period of "power off", even hear power off noise from hard disk(s).
(3) As you described, power button seems not working.

I am still puzzled by the behavior (This is the first case such symptom is reported), because:
(1) Power button is not stuck permanently, otherwise, NAS will stay off after power off by software when stuck button is detected. You can verify this by press and hold your NV+ power button, system will be off and keep off.
(2) Power button is not stuck for longer than 10 seconds. If power button is held long than 10 seconds, then it should boot into one of state that power button will be ignored that you won't be able to see reboot pattern.
(3) Power button has to be stuck for longer than 5 seconds to be recognized as valid power off request.

So, when you plug in power, power button is generating repetitive pulse that is longer than 5 seconds but shorted than 10 seconds. Sounds to me that one of two capacitors related to power switch is faulty(one is for power button de-bouncing. The other for keep NV default off when you plug in AC, this is the one more likely to cause your problem). Could you please do below experiment: Press and hold power button when NV does this 2 minute reboot cycle(Why 2 minute? because that is when hotplug agent is loaded and detecting power switch), does it change its behavior. Normally, when you press and hold power switch during boot time, all disk LEDs blinks every 5 seconds and enter into different boot mode depends on when you release the power switch 5th second(Skip Volume check) 10th second (TFTP) 15th (Tech support) 20th (USB boot) 25th (memory test) 50th and above (TFTP).

Depends on outcome of your experiment, we can identify which capacitor is probably guilty. The fix is to take it off(or simply cut it).
The software work around is: Don't load power switch hotplug agent. The draw back is that you have to shutdown from FrontView. (When you put NV hard disks into NV+, you can ssh into it, modify hotplug load script, or kill it if it is loaded, after change the scripts, move them back to NV. Software will then ignore power switch no matter how faulty it is).