You are correct that B12 is absorbed in the ileum and we retain a store that is enough to last years. But, we do still have our ileum. It is only the colon and rectum that is removed. Some people may have some ileum removed if they have a lot of backwash ileitis damage. It is the terminal ileum that important for B12. This is the distal several feet. The pouch itself takes only 6-12 inches, and should absorb nutrients unless there is pouchitis, or there is a very high output. Another factor is if you take a proton pump inhibitor chronically. You need stomach acid to absorb B12. So, it is not that all j-pouchers have trouble absorbing B12, but some do. I developed B12 deficiency after about 18 years with a j-pouch. But, I also am over 60 (it is more common as we age). My husband is deficient also and has his colon.
In regard to the extremely high B12 level, it could be lab error, or you are just taking way more supplement than you need. My B12 went way high when my doc prescribed 1000mcg a day. I now just take it twice a week, and it is in the 300-500 range.
Paul, your GGT isn't too bad, especially since it is stable. Hopefully, you are abstaining from alcohol for a minimum of 24 hours before your lab tests, as it can artificially elevate the value beyond whatever liver inflammation there might be. Still, reducing your daily intake would be a good idea (2 drinks per day is recommended for the average male, and you are drinking twice that). Even reducing a little could make a difference.
Jan