There's a difference between soluble fiber and insoluble fiber. The insoluble fiber that you eat stays in your digestive tract - so the doctors are correct about that. However, that's not the only kind of fiber. Soluble fiber is absorbed into your bloodstream and travels everywhere - it can (and does) clean circulatory systems (reducing cholesterol and allergies), gets into lymphatic systems, and can get into breast milk with positive results. It is difficult, but not impossible, to get both in the same product that will also reduce reflux.
I'm shocked that anyone would tell you a breastfed baby doesn't poop very often. I think it's well known that it should be the opposite! Most breastfed babies are pretty famous for pooping the second they finish nursing, or two seconds afterwards!
Drinking water should not end up in your sinuses. The trick is to have it absorbed into the cells without being refluxed up your digestive tract. That's simple to solve with a patented hydrating agent and/or digestive herbs in a patented and balanced formula. Please realize that it's extremely difficult to get a patent on a food product - very few products qualify. A patent also signifies that the product has been proven safe and unique, as well as effective. That's important for all of us, but especially for pregnant and nursing moms and for infants.
I have a good friend who had terrible digestive issues and constipation, and her newborn had them as well. She switched to patented products for herself, which relieved the problem in her baby. When her baby started to come off breast milk, she got the children's version (again - patented!) and had no problems. Baby #2 had no issues because Mom was already stabilized. No mama and no baby should be on drugs or miralax or anything else when there's something safe developed by a lead scientist in pediatric nutrition. What my friend used is also used in feeding stations for severely undernourished and compromised children, supervised by physicians. You want something manufactured in the US and heavily inspected.
It's not going to work if you just eat more lettuce and brown rice! That sort of fiber is not going to get through to the baby. Let me know if you want more info - I consult all the time with moms of kids/infants.