So an update. The day before I took the motherboard in nothing would come on, not even for a second, but when I checked the PSU separately the fan would start up. I took the mobo to the service center, they checked it, cleaned it, replaced some part of it and it booted and reached the BIOS with their PSU/CPU/RAM setup.
I brought it back, reseated the CPU, thoroughly cleaned the CPU fan, re-applied thermal paste, connected the CPU fan, one stick of RAM and booted. So it booted up(unlike before I took the mobo in) but only for 10 seconds(nothing on the display) before powering down and then and endless cycle of the same powering on and off. I re-checked the standoffs, checked all the cables again, moved the PSU out as far as the cables would allow me and cleaned it, and it finally booted, both with OS X and Windows. So then I put in the 2nd stick of RAM, put in the GPU and try again; once again, it boots both into OS X and Windows with no issue.
Finally, I re-connect the side panel fans(and may have potentially brushed some other cable with my hand), put the panel back, press the power button, and am just about to clean up, when I realise it's doing the 10 second endless boot cycle shit again. I rebooted after disconnecting all the case fans, and it still refuses to get out of the boot loop.
I'm really lost at this point. Should I take the PSU home and check it with my setup, or could it have anything to do with the cabinet fans?