I am hoping to process a few hundred records this weekend, which just means going one by one and adding tags and notes. LMK if you recommend a better method, but what I plan to do is have two windows open, EagleFiler and the info window side by side. It seems the keyboard navigation shortcut to move up/down sticks on webarchive records that seem active (pop-ups, etc). Even so, I still need to mouse back and forth between the two windows and specific fields that I want to update. Maybe you could think of a way to use keyboard maestro or stream deck to make this monthly process less carpel tunnel and tedious?
You should be able to stay in the Info window and use the Go ‣ Next/Previous Record commands (Command-Control-Up/Down).