Hmmm, I am wondering if perhaps coccidia may be an issue. This protozoans is pretty difficult to detect with normal stool sample and may cause persistent diarrhea. You can read more about it below:
COCCIDIOSIS: DIAGNOSIS, TREATMENT, AND PREVENTION
Treatment consists of antibiotics.
I think it is pretty odd that the diarrhea persists with all the tests and treaments, maybe you can get a second opinion? IBS may be possible but i think you would have seen some improvemnt. Just my 2 cents!