This process of testing EPD panels without connecting the battery, frontlight, touch, and/or pen input (depending on model) is basically the same process used by suppliers of new-old-stock EPD panels in China, when they sell them on online stores like AliExpress and/or eBay, which you would've most likely purchased your display panel from.

However, they would sometimes test displays on another device that uses the same display or used a display with similar specifications, in order to not have too many test devices around, for example, a Kobo Aura display tested using a Kindle Paperwhite 2, or a Likebook Mars display tested using a Dangdang Reader R7.

At this stage, you should not have the infrared light guide attached to the panel but not to the mid-frame, which is what the photo shows as I made mistakes while writing this guide, so that you don't have to.

Don't mind the weird selection of books in the middle row, but I had not removed the previous owner's sideloaded e-books before carrying out this repair.