C., my daughter was diagnosed with Williams at almost a year old, so I know what you are going through. At the time, we had several different syndromes that it "could" have been. I believe one of them was Turners Syndrome, and there was a couple others. I struggled with doing the genetic testing as well because I thought it was just milk allergies (we tried EVERY kind of formula on the market, even the specialty mail-order ones that cost $300.00 a box!!!) - that didn't help. We considered every other possibility until finally my pediatrician convinced me by saying that if she DID have something they need to know because there are a lot of health problems that go along with these syndromes that need to be kept up on. So I broke down and got her tested, and low and behold she had Williams. But since finding out for sure what she had our lives have changed so much for the better because we are now meeting other Williams families and getting support from others who went through the same things :) So my advice would be to get yourself a geneticist referral and make a list of all the things your daughter has had happen to her and what she is doing now. Take that in and get the tests run - if they come back negative that's great, but if not then at least you know what to expect :) It's helped us tremendously knowing what we might have to deal with in the future and look for. Strange story too - when we walked into the geneticists office at Riley Hospital in Indianapolis we were standing outside in the hallway with our daughter waiting for our room. A doctor walked past us and just briefly glanced at Abby (my daughter). He didn't stare, just looked like you would look at a cute kid LOL. A few minutes later the other doctor came back into the room and said "the doctor thinks she has Williams Syndrome." Just by looking at her he knew with 98% certainty that's what she had. And although she does have the facial features, you can't actually tell that she has a syndrome because she doesn't look like it that much. She hasn't quite "grown into" her features yet I think. But just like that he knew what she had, and we ran the test and a week later it came back positive. So going to see a geneticist may be great too because they deal with all of these different problems on a daily basis and they just know what to look for, so they might be able to narrow something down and rule things out much more easily than a regular doctor or ped would. So go, have it done and take the stress off of yourself, you will feel much better knowing for SURE that she either has it or doesn't.
As for the constipation issues - Abby had those from the moment she was born as well, she would go for days with nothing and then EXPLOSIVE poops that were very difficult for her to get out. My pediatrician recommended Benefiber, start her on 1-2 Tbsp a day and go from there. Within 2 weeks Abby was better, and she hasn't been constipated since then, and that was over 3 1/2 years ago. I have tried stopping the fiber, but then the constipation comes back (symptom of Williams). It's perfectly healthy and natural, doctor said they could take the whole big bottle and all it would do was make her poop poop poop. She has never had any problems with it, I buy the Walmart brand for $10.00 and it lasts me almost 2 months. I just started with the 2 Tbsp a day and then upped/lowered it as needed until she was on a good schedule. I mix it in with her cereal she gets at night and she doesn't even realize it :) So you can try that, I have told a lot of people about it and they have all tried and said it worked great!!! Worth a shot. Good luck hun, and if you ever need someone to talk to or help you can email me ____@____.com :)