I'm not sure why the PCIe spec says only use 2 pins each for power. Both the Corsair PSU and AMD Radeon adapter cables I have both use all 3 on both sides. So I guess there must be some optional use or something.

I would recommend using 2 wires at least even if jumped between pins on the wires. I'm more concerned with all the current flowing through 1 pin on the PCB joint to copper. It's likely within ratings but will cause more heat than spreading it over 2 or 3 pins.

That was easy. PIC programmed and working as expected.
Time for new snapshot. A few things to notice:
I don't have a spare PCIe cable and don't want to shut down mining so I just soldered on power leads. The glare circle is from my ring mag light I need to actually see pins when probing. You can see the green power LED, and when I send a ktest cmd the red LED blinks. The 330uF caps are too small - that's because I got them locally and they ran out of the 10mm ones. And finally at the top of the PIC you can see the power fix lead tapping onto the nearest 3.3V by the NOR gate. It's the little gray worm-like thing.

You should expect around 40W (3.3A) . It probably will use less. Using the max ratings for regulators, under a condition of over clocking, you could probably draw up to 55W (4.6A) beyond which it would be dicey and shut down should occur if the regs are doing their job. Off hand, I didn't go check at what rating the shutdown  kicks in. This is allowing an approximate 75% for regulator inefficiency but actually hope we get better.

The PCIe connectors are actually rated at 13A per pin by Molex and I guess the PCIe spec takes a pretty conservative approach. If you used sufficient wire thickness to avoid heating I think whatever your PSU is capable of could be split onto multiple boards. Most PSU have pretty hefty 12V rails available between PCIe and Molex. You can likely even find 24pin mainboard adapters letting you use that source as well. I think I've seen something like that on ebay. The cheap Molex-PCIe adapters tend to use wire that is a bit thin so you wouldn't want to split twice with those.