Option 1: Unplug battery from circuit board, and unplug usb plug. At this stage, the phone is off. With battery unplugged, press VolumeDown button and plug in usb cable (the usb cable must be powered either from charger or computer). The phone should start in bootloader mode. If you want to enter recovery, you must plug back in the battery connector to the circuit board, other wise the phone will just turn off. Then proceed with ADB/Fastboot as you suggested. (Tested this method right now to make sure it worked before posting)
Option 2: You may have a chance a starting the phone by short circuiting the pins to which the PWR and/or VolumeDown buttons are soldered to on circuit board.
I tested this method because my PWR button was broken and was acting like it was always pressed which forced a reboot each time. I unsoldered it and dismantled the button, clean inside it by scrapping off dirt and recreating the bend or dome shape of the button mechanism. I then finished by re-enclosing the button and then soldered it back to the circuit board. It worked fine, until next morning upon wakeup I decided to resolder the button because it wasn't perfectly parallel to the side of the board. By re-soldering, I destroyed the solder pad and was never able to solder it again. The phone is not usable as a daily anymore, only usefull for surveillance cam or server...
Edit: found this thread that may be usefull for you.