Ss- I have to say- you have a knack for writing posts that tend to stick with me and that I brood about for a while.
I'm sorry about the challenges your daughter is facing. Being gifted at that age is its own special kind of burden. I doubt I was anywhere near as gifted as your daughter, but I also read at 3 and was reading college level books in 2nd grade. I went to a parochial school so there weren't any special programs for me and I was literally bored to tears a lot of the time. My peers were pretty ok, all things considered, but I was taunted a lot about being a "walking encyclopedia" and honestly, while I had friends, it was pretty lonely and alienating at times. I was just a little too "different" and couldn't ever quite understand why the other 8 years olds weren't reading "A Tale of Two Cities". Books were such an essential part of my existence and there was no one to share them with. I acted out by absolutely refusing to do any homework and just acing all the tests. My parents were at their wits end. Then, much like Rpp's daughter, I got to High School (a public one), got into advanced courses, and discovered that there were other people in the world that were similar to me. A lot of people have problems in high school, but for me it was a blissful time, because I felt comfortable in my own skin for the first time and didn't feel I had to "hide" who I was. I absolutely blossomed. I wish that for your daughter (but hopefully well before her teen years).
Like many others, I am keeping my fingers crossed for the testing.
And now to practical matters. I would suggest keeping a journal of all those negative behaviors your husband displays towards your daughter (snapping fingers, threatening boarding school, making comments about eating). You will probably never use it - but I can think of a few circumstances where it might end up coming in useful. Worst case scenario - custody dispute. But possibly you might end up in co-parent counseling someday and having concrete examples of behaviors your H might not even believe he is doing could be helpful.