Is my PSU the cause of apparent voltage fluctuation in CPU-Z

no, that is your cpu or motherboard. Im not familiar with amds, but intel cpus have "vdroop"/"load-line calibration", and it protects the cpu from voltage spikes and dips by artificially raising the voltage under load. Also, with the i7 cpus, when the cpu gets too hot, it raises the voltage to keep it stable. A slightly higher voltage reading when stressing the cpu is normal, like 0.02v, but im a stranger with amds so hopefully somebody else can jump in

I had this problem before. It stopped when I got a board with VRM coolers. Try buying some ramsinks or minheatsinks whatever they are called and sticking them onto the VRMs.

1. Like MrLinky said, vDroop/Load Line Callibration. In the BIOS, it may have a setting like vDroop: enable/disable, or LLC enable/disable, if you enable Vdroop, LLC is disabled, and if you enable vDroop, LLC is disabled, they are like two sides of a coin as far as the BIOS setting.

2. There will be the actual vCore, say, then there is what you set in BIOS, and what the software thinks it is. These are usually all different, from my experience comparing them with a Fluke digital multimeter.

3. From what I have heard, you may indeed want to have vDroop enabled at that vCore.