I just wanted to encourage you in the fact that although he's so little, believe it or not, it's probably going to be a little easier to get him adjusted to gluten free eating. My son was a year and four months old and he is now 3 and eats most any gluten free pasta and/or bread. For one thing, velveeta is gluten free and even gluten-free pasta covered in it makes some good mac&cheese! That's what we do at our house and even my 6 yr old likes it(her and I are non-celiac). A bread maker with gluten-free bread mix makes the best bread ever! Again, we all eat and like it.There are gluten-free waffles that can be found at most health food stores. I don't know the brand but there is a gluten-free cracker that isnt too bad. Ball Park hot dogs, any HillShire Farms meats, Cheetos, Cool Ranch Doritos....all gluten-free and my son likes them all!! I don't know if you are in an area that has WalMart close but their Great Value brands of food are very good at labeling gluten-free any item they have that are gluten-free. Cocoa Pebbles, Fruity Pebbles, DynoBites, Little Einsteins, and Trix are all gluten-free cereals that can be found at the grocery store.
Also, something that I've always done with my son is told him the reason he couldnt have a particular food. I would tell him(even at a year and a half) that it would hurt his belly or make him sick. As he gets older, we're getting more into the discussion of Gluten. Now it's more of.. no its not glutenfree or that has gluten in it. He understands the concept of a belly ache. And he understands that there are some foods that hurt him. As a former preschool teacher(in case that counts for anything!), a mom of 2, and a celiac mom of almost 2 yrs, I would say that the more that you explain(on his level of course) WHY he cant have it the more he's going to want to avoid those foods. Kids are alot more resilient than we sometimes give them credit for.
As far as the scope goes, I think that if the symptoms are there but tests are coming back negative, that's the next step to confirming it as celiac. For my son, they had both scheduled even w/o results so we could be certain. His blood tests were inconclusive but his scope revealed the thin, damaged intestinal linings. His pediatric GI has already told me that at 5 or 6 he wants to have him get back on gluten then have another scope to check the results. We didnt really go into alot of that but I assume it is bc its pretty rare to make an "official" life long diagnosis at 1!
Also, in reference to the breastfeeding scenario, I would think that at only 4 months shy of being 2, you could safely wean him but like you said, so many transitions. Maybe they could schedule the testing for a little ways away to give you time to get him weaned and then get back on gluten before hand.
Our case was pretty traumatic. He was so little, eating like a pig, withering away... it was awful. But over a year and a half later, I'm the voice of hope... He is happy and healthy. Right on track with all physical and developmental milestones. Not a bit of diarrhea, no tummy aches, no gas and bloating! All the time it gets a little easier, we find a new gluten-free something we thought was off limits forever! I hate that getting him diagnosed was so traumatic but I'm so glad you guys are on your way to good health!