This is all too common. Disable the eMMC, boot to microSD, enable the eMMC, and erase it. There are numerous posts about this here. Unfortunately they aren't easy to find.
Basically open the lid 90°, hang the display over the edge of a table, and take the bottom off. Switch off the eMMC. Turn the power on. If you have to move the PBP, only pick it up by the display half, and don't turn it right side up or pieces will fall out. When booting has started, turn the eMMC back on and put the bottom back on. Then you can erase the eMMC and/or install the operating system of your choice, with an appropriate U-Boot or Tow-Boot.
Basically open the lid 90°, hang the display over the edge of a table, and take the bottom off. Switch off the eMMC. Turn the power on. If you have to move the PBP, only pick it up by the display half, and don't turn it right side up or pieces will fall out. When booting has started, turn the eMMC back on and put the bottom back on. Then you can erase the eMMC and/or install the operating system of your choice, with an appropriate U-Boot or Tow-Boot.
:wq
