God this is a solution beyond what I expected! It is working perfectly in v.1.2.5. Thank you so much!
Oh, whoops! Yeah, the plugin is trying to make sure you don't select the Item button when in discard mode, and since the item button is between the cards and the Discard button you can't reach the Discard button.
I've written a fix but it has one small issue: The item button can be highlighted (but not activated), just as a necessity to get to the discard button using the keyboard. Hopefully this is fine for you.
Here's the new version of that plugin. Once again, let me know if there are any issues aside from the one I just mentioned.