I wasted a few hours trying to bring an ac68u back to life the other day. I eventually had to bite the bullet and do an nvram erase via serial. Then recovery mode worked and I could flash with the asus firmware restoration tool...
This is absolutly brilliant!!! My router was in the same exact boat as yours, so I had to create an SNB account just to thank you for your amazing write-up, pictures, and ultimately reviving my router!
For those on Windows 10 64-bit, I was unable to get PuTTY to cooperate (the open button...