I think that people are going to have different opinions for two different reasons:
1. They have children with celiac and have been through the testing before ( some with a co-parent).
2. The degree in which they are sick if they are Celiac themselves.
I don't know much about Celiac and it's testing, I was just diagnosed two months ago. I was a lab tech for 16 years but didn't work in immunology so I didn't deal with a lot of testing dealing with autoimmune disorders. I know different labs use different methodologies and instrumentation so that could be why the normal values are different. For Mayo in Rochester the values are <4 - >100. I don't know if my doctor would consider a 12.6 increase from the normal range significant or not.
I can't state if I would be comfortable with the waiting six months, but I would be asking questions like:
Should we be trying to figure out why her tTg IgA is elevated ( if even only slightly) when only disorders and conditions like chronic liver disease, diabetes, crohn's, colitis, thyroiditis, and serious infection can cause the slightly elevated test result if it isn't Celiac? Won't these conditions and disorders also cause long term health problems ( if not death) if not caught, or is it fine in 6 months to do that testing also? One lab draw would at least rule out the liver disease, diabetes, thyroiditis, and infection. Those would be the ones to worry about waiting 6 months for a diagnosis. Just my opinion.
I understand you wanting to wait since you have problems with the co-parent and I feel your pain and I am glad you are comfortable with it. Thankfully when I called my ex to first tell him I wanted the kids tested he didn't fight me on it, and when I called him to tell him my 11 year olds test was positive he let me take the reigns and help him figure out to go gluten free( which I wish I wouldn't have done and waited until her tolerance test but I didn't know she needed one). I asked the GP if I should have her gluten free and he told me yes. I didn't now until the ped's GI called two weeks after her diagnosis to schedule an appointment that they want the endoscopy. The said we would discuss it at her appointment if they want her to do the challenge, by then it will have been two months for her gluten-free. I had a biopsy and blood drawn all on the same day and didn't know just how they went about it in patients that weren't in the hospital. But then again I think he gave in so easy since I have been sick for years and am slowly wasting away still even after a diagnosis, he sees it.
Good luck to both you and your daughter and Welcome to the club. I hope you get the answers for her and it causes the least discomfort for her and in turn you. It's rough to see your children sick.