There isn’t a lot out there for what the LED codes mean. A similar question was posted last month (
https://www.mobileread.com/forums/sh...d.php?t=359026) with no replies. Perhaps you could post about it in the dev forum.
Since the Kobo Clara HD doesn’t boot with your old battery either, and you’ve changed SD cards, you could have damaged a component on the motherboard.
Worst case scenario, you could hunt around online for a Kobo Clara with a broken screen and Frankenstein it together with your Clara to create a new, working one.