along the juices line, have you tried soups that have been heavily blended? If starches are working well for you, maybe you should try making some potato/sweet potato/squash type soups, and blend them for a while. Also, for added calories: use some heavy cream in the process. You can make them taste well, and hopefully make them easier to digest.

As far as vitamins go: I'd suggest that you start taking them. Although you are suffering from something unknown, this is sort of similar to people that get gastric bypass surgeries, in that when the food passes through their system their new stomas aren't able to absorb the nutrients as well and due to this, have to take heavy amounts of supplements. I'd suggest a strong multi vitamin, along with vitamins c & e, flax seed, and calcium...or just start out with the multi vitamin and see how it goes from there.