ValarMorghulis,
Sorry to read that you're still having the troubles.
Yes, given the recent history, there's a strong possibility that the motherboard is defective. I think of Gigabyte as having good performance and quality but a quick look at user comments on Gigabyte Z87 LGA1150 boards on Newegg and Amazon showed is a reasonable proportion of DOA boards. Of course, user comments are much more likely written by those unhappy with a product than those satisfied, but with boards with a small number of comments, there were a couple of Gigabyte boards with 20% bottom ratings.
Faults in the motherboard power chain could be revealed if there are test points and you can measure the current at those points - I suspect the fault is very high up in the chain > there may be failed/shorted capacitors or fault in the connectors such that no power is going onbaprd. Are there any instructions in the motherboard manual that describe testing for faults? Does Gigabyte have telephone or chat help?
You mention an additional standoff, and this makes me think of problem associated with a motherboard that is accidentally grounded to the case, and there are examples where standoffs do this with a result similar to yours. The conventional way to eliminate this possibility is to mount everything outside the chassis on a test platform or some have make-shift wood frames- just create circumstances in which all the components are not touching any metal, are near enough to each other, and secure from moving around. There have to be YouTube videos on doing this properly. If you try this, be methodical and watch- and listen carefully. If the system works- fantastic, but there is the chance that you could set this up and there will be a little popping and smoke, which means that something was shorted earlier.
My guess at this point is that you probably will be trying another motherboard.
Cheers,
BambiBoom