I don't see as any damage was done. The psu side sockets are all the same, not similarly keyed, same. Doesn't matter if you plug in eps or pcie, that's why it's printed as pcie 6+2 & cpu 4+4. There's actually 5 interchangeable sockets, the upper single socket is upside down from the lower 4, but the pin layout is also upside down, making it the exact same thing.
If you are having issues, it's not due to power connections.
I'd start with dealing with 800x600 for now. Get online and goto guru3d.com and download DDU. Run it in safe mode. That'll delete fully any gpu drivers. Goto piriform.com and get ccleaner. The free version is fine. Run it. Several times or until it comes up empty. Run the registry tool, (say Yes to backups!). In windows, Win Key + X, Command Prompt (admin). Type cd\ (enter), then SFC /SCANNOW (enter). That'll run through your windows and repair any system files that might be buggered.
Then download a fresh copy of your gpu drivers and try and install.
But none of that will fix bios issues, as bios is totally seperate from anything that comes after, all of which is software. Many recent motherboards have dual bios, A/B switchable, might see if that applies.