How do I prevent my cd ripper from overwriting Unknown Album when metadata can't be found

You could automate the process by wrapping the abcde with a script that invokes abcde and checks for 'Unknown Artist/Unknown Album' directory existence after abcde completion.

If the test shows that directory exists then rename it with a name that includes the disk CDDB ID which you can get with cd-discid tool. This script obtains the ID early, otherwise auto-eject prevents obtaining the ID after abcde has completed.

#!/bin/bash

id=$(cd-discid /dev/cdrom)
abcde $@ || exit 1
if [ -d "Unknown Artist/Unknown Album" ]; then
    mv "Unknown Artist/Unknown Album" \
       "Unknown Artist/Unknown Album $id"
fi