A lot of what happens with surgery is going to depend on the results of your MRI: is it a complete or partial tear and will your surgeon be able to fix it laproscopically or with an open incision? And of course, which arm it is.
Mine was a massive complete tear, on my dominant arm, and had to be fixed with an open incision, so my experiences are probably a worst case scenario. But I made it through surgery and recovery without regaining any weight (I was in maintenance) so I count it as a success.
The surgery was out patient and they used a nerve block, which gave me complete pain relief for 12 hours. After that, I had three kinds of pain pills and really needed them. The first week was rocky, I won't kid you. I woke up in a gigantic velcro sling and was not allowed to remove it for six weeks, except to shower (after a week) and go to physical therapy. I was expressly forbidden to do anything to fire the muscles on that side for the first six weeks because the tendon in my shoulder (screwed back on the bone with titanium screws) had to reattach. I wasn't even allowed to use my fingers on that hand.
So I learned to do everything left handed. Including mascara and eyeliner, putting on stockings, eating, computer -- everything! It was easier than I thought to become ambidextrous and to this day, I still use the computer mouse with my left hand because I learned to like it that way after the surgery.
Like you, I was panicked about weight gain, so I focused on food and exercise. My doctor was a big proponent of me doing whatever exercise I could (that didn't fire the muscles of the repaired shoulder) because he said getting oxygen circulating would help the healing process. So after the first week, I did 30-60 minutes on the recumbent bike every day. I also lifted weights with my left side because I had read some studies showing that there might be a reciprocal effect on the unused side. At least it kept one side in shape! And I worked out legs, carefully. I couldn't do elliptical or walk because the bouncing was painful, but the bike was perfect cardio for recovery.
In PT, they only stretched my arm to prevent scar tissue from forming for the first six weeks since I wasn't allowed to use the muscles. It hurt like a son of a gun but at least I was able to regain a full range of motion. After six weeks, we started with resistance bands and tiny little Barbie weights (which kicked my butt!) I'd say it took a good four to six months for me to build enough strength to be back to fairly normal weights on the repaired side and be pain free. You'll be taking pain pills before your PT sessions long after you don't need them for anything else and you'll hate PT because it hurts, but it's the key to a successful recovery.
As for food, I kept on eating like I always did, with an emphasis on protein for healing. Nothing changed with regard to my eating plan: I still counted calories, used Fitday, planned my meals in advance etc. I didn't lower my calories despite getting less exercise because I figured I needed some for healing, but I didn't raise them either. I froze tons of food in advance, both for DH and me, in individual portions - salmon, chicken, soups, chili, burgers etc. There wasn't anyone to cook for me but it wasn't hard to cook left handed -- it just took twice as long
(everything takes twice as long). When we had to get groceries, DH would drive us there and man the cart. We managed without any problem during my recovery and never ended up resorting to pizza!
It took about six months for me to feel back to normal, but the roughest time was the first six weeks when I wasn't allowed to use my repaired arm at all. When you're about three months into recovery, you start to wonder if it's ever going to get better and the answer is yes! Just hang on and the day will come when you're pain free and will have a complete range of motion back. In the end, it's all worth it!
What other questions can I answer?