I think from what im reading is that you did both options at once and it worked? If so,leave the ACC and 12v plugs and simply plug "Power on" jumper cable back into psu and try boot again. If that works then it was just the ACC and 12v mixed up and you should have your automated start up/shutdown again. If it does not boot, then there's a problem somewhere on psu. So the only other way around it which is the set-up i use.
Install an on/off (momentary/toggle) switch on the vehicle's dash or a convenient location and plug it straight into motherboard PWR jumper pins and bypass the psu alltogether. Then get into Windows and change the power button settings to suit hibernation/sleep when you press on/off switch. Also, remove jumper pins on m2 atx so its in normal psu mode as well as the ACC plug(no need for ACC). By doing it this way, it will give you more manual control over pc. E.g. if pc freezes you just hold the on/off button for a few seconds to turn it off and then press again for reboot. Turn the pc on when you want it to so if you just on a breif trip to the shop the pc wont automatically boot up only having to shut down a couple mins later. Hopefully it was just the ACC and 12v and that youve fixed problem. Good luck