I wish I had answers for you but is there a therapist or counselor that you see by chance that you can talk to about the SI so that they might be able to help with these questions. If a doc has put you on a specific diet and calorie restriction, it might be possible to get them to order a nutritional plan while you are there, just like they would if you were diabetic or had high blood pressure. Talk to your doctors about this and see if they can help. Otherwise I would work with what is given to help heal as there are specific foods following some surgeries that are more beneficial that others and the nutritionist at the facility knows this. See if your docs can't work with them.

Meanwhile, definitely sending good wishes your way that the procedure goes well and for a speedy recovery!