Your liver makes bile, a chemical solution that acts like dishwasher detergent. It surrounds the fat you eat and breaks it up into small particles you can absorb through your intestinal wall. Bile is stored in your gallbladder, when your stomach distends from eating a chemical signal called cholecystikinin (CCK) makes your gallbladder contract, like squeezing detergent into the sink. The tube that the bile flows through is the common bile duct, which dumps into your small intestine just past your stomach. In most people, right before the papilla of Vater (the end of the common bile duct) the pancreatic duct connects so that pancreatic juices empty into your intestine as well.
If bile is like detergent, pancreatic secretions are like Drano. Your pancreas secretes a number of digestive enzymes, little chemicals that act like knives to chop up proteins and carbohydrates into small enough pieces that you can absorb those.
Stones, or crystals, form in bile because of a variety of reasons. Bile does not stay in solution (100% liquid) well, so if there is a relatively small variation in the three components that make up bile you can start to form crystals. If your gallbladder does not empty well, you can start to form crystals. If those crystals get big enough you end up with gallstones.
If the gallstones block the common duct, you can run into a couple of problems. One is that because you can't absorb fat well from your small intestine, the fat makes it to your colon where the bacteria are more than happy to take in the fat as food. The change in normal bacteria counts in your colon, or that change in the relative numbers and types of bacteria can cause inflammation of the colon, which leads to pain and diarrhea. Your colon's main job is to remove water and some salts from the tea-like stuff that comes out of your small intestine, diarrhea is usually a sign that your colon is not absorbing enough water. If you cannot excrete bile, you will build up one of the components of bile called bilirubin and can develop jaundice. Bilirubin is what's left of hemoglobin after red blood cells are removed from circulation and broken down for parts in your liver. The bilirubin is excreted in bile and eventually in your stool.
The other problem you get from a stone in your common duct can be pancreatitis, inflammation of the pancreas. The pancreas makes digestive enzymes, if the pancreas cannot excrete these the pancreatic enzymes will begin to digest the pancreas itself. This is bad. Also, if your pancreatic enzyme level is low in your intestine you will have trouble digesting proteins and carbohydrates, which can lead to malabsorption and mess with your colon just like too much fat will.
If you have had stones in the Common Bile Duct, that could be a reason for a scar in your CBD that presents as a stenosis. Ultimately, they're going to have to do an ERCP, which is like an MRCP but with an endoscope they put through your stomach and into your intestine. Don't worry, they sedate the stink out of you and you won't remember anything about it. MRCP may miss tiny stones, they will probably inject some x-ray contrast and take a picture, and if they find a stone they will grab it with a basket or sweep it out with a balloon. They will also probably take brushings of the inner lining of your bile duct to be sure it's benign.
It's very good that you have no evidence of a pancreatic tumor. Those are exceptionally nasty and you don't want one of those. Your docs may talk to you about a cholecystectomy, an operation to remove your gallbladder. Stones in a gallbladder means it does not work normally anymore. Small stones are actually worse than large stones because small ones can pop out of the GB and get into your common duct -- and cause exactly the problems and imaging findings you are describing.
Edit to add: if there is a significant stenosis, the gastroenterologist may well leave a stent (a little plastic tube) in the bile duct to open it back up. If this relieves your problems, they may go back with a metal stent - the kind that is used in coronary arteries. Actually, vascular stents are only approved by the FDA for biliary stenting, but they work in arteries and are used there "off-label". Many of your symptoms may be due to chronic pancreatitis.