JOM, I think getting a fresh set of eyes on your situation is a good idea. Hopefully you have already consulted with a hematologist, who can better sort out the reason for your iron depletion.
To clarify what you said, the ileum is in fact part of the small intestine. It is the longest part of the small intestine- about 2/3 of it (with the preceding sections of jejunum and duodenum being rather short). The ileum is about 11 feet or so, and attaches to the colon at the cecum. You may well have had some ileum removed with your colon, but not all of it, or you would be losing weight.
The terminal ileum (the last few feet) is where B-12 is absorbed. Iron is absorbed in the duodenum and jejunum. Iron absorption is a complex thing, affected by many factors. I see that your docs have ruled out Crohn's, but have they ruled out celiac disease or gluten intolerance? Another common problem is chronic use of proton pump inhibitors, because reducing stomach acid too much will block iron absorption. Anyway, lots of factors there. It does seem that absorption is one of your problem, since iron infusions takes care of things. http://www.hematology.org/Pati...Iron-Deficiency.aspx
I hope you get some answers soon!
Jan