Put the console to charge for a couple hours , after that try to enter RCM.
Please understand your problem is you can't enter RCM.
The main issue is that you did downgrade and fuses doesn't match, so you can boot only through custom bootloader (hekate , fusee-primary).
The second issue is that you did downgrade without system reinitialization, that's why the console hungs on boot even with "custom bootloader".
Now to get the console working you need : 1) Enter RCM. 2) send Hekate and restore your backup.
that's all ! Don't search any other guid , waste of time, all returns to RCM.
You don't need any keys wtf. stop right there and do what people are telling you !