My experience with battery life seems to be the opposite of yours. I'm using it on a Kobo Clara BW and my battery just lasts and lasts, even with an hour or more a day of reading. I don't have any firm data on it, but I know that right now I'm at 52% battery and it's been at least a week and a half since I plugged it in anywhere.

I use KOReader a lot. After your comment, I realized file browser annoys me too. After some searching, I found an alternative view. Turns out, you can open the File browser tab, swipe down and switch "Display mode" to "Mosaic with cover images". Also seems like it's default for new versions now. This mosaic view still uses directories but it lists books as rectangles (covers) instead of lines (covers + titles), it seems more pleasant.

By the way, in the same settings I finally found how to set up KOReader to launch with a previously opened book. What a relief!