Ok, So I finally found a working path to upgrade my v11 database to v12.
It was much simpler than I had expected.
All I did was:
- force restore the v11 database (with files) into the v12 clean system
- ran “bench migrate”
- After the successful migration, I replaced the encryption key in site_config.json with the one from the original v11 site
It was that simple. The downgrading and upgrading path had far too many errors and ultimately would not work at all.
Hope this helps someone else.
As always… Your mileage may vary… 
BKM