If you are concerned that the doctors could be too pessimistic, suggest that
your MIL geta second opinion. You don't say where you are, but this cancer is rare, and some doctors have very limited experience with it. Even the ones with experience may see only 4 new cases a year. And that's in a busy cancer center in a major metropolitan center.
Having said that, you should know that the cancer, when advanced is often considered "unresectable" (Medical lingo for "we can't surgically remove the cancer.") It sounds as if that's what your MIL's doctors are thinking.
In my case, the doctors went in to relieve a blocked bile duct, realized there was cancer there, removed my gallbladder because it was a mess, and also spotted a small growth they felt (correctly, as it turned out) was cancer on my intestine. Because of the way they calculate these things, that made my cancer stage IV, having metatisized. So they did not remove the cancerous bile duct, but left it in. Later pathology tests indicated that the cancer started in the gallbladder.
Through a process I still consider amazing, I had no immediate recurrence of the cancer after the surgery. Do I wish they have removed my bile duct? Knowing (now) that my cancer is slow growing, I do, but I can't go back in time. Had the doctors known I had cancer, they wouldn't have opened me up at all to relieve the blockage, but would have taken other steps (probably a stent). When I did have recurrence of the cancer this past January, in the area where my gallbladder used to be, I wanted them to go in an take the tumor out. I completely understand your feelings about that! However, they don't do surgery in a situation like this, and even my highly sympathetic surgeon whom I trust and value, would not consider it. If the tumor created a blockage in my colon, he said, he would go in and remove the blockage, but he would not touch the rest of the cancer.
I share my story to empathize with what you and your family are facing. This is such a hard cancer; it's hard to get information, it's hard to find doctors who know enough, and it's so hard to see otherwise healthy, functioning adults suddenly brought low by this aggressive and dangerous disease. I do suggest that you learn everything you can, so that you can feel like a support to her and the rest of the family.