This is a draft, please do not link to that page yet.
This is a list of frequently asked question.
If your battery is stuck inside the pinephone, grab a screw driver and completely unscrew all the screws of the midframe. Then pull out the battery (you may have to fully take off the midframe in some cases to get it out)... And then rescrew the midframe, but only tighten the screws to the point where they are just barely tight to hold. This should allow you to remove the battery easily.