I'm not sure I have any great supports beyond what you've written, but I do think that the best evidence is the one line you added "other suburban districs allow this."
In our district, freshmen can take one of four choices: World Geography, Honors World History, AP World History or AP Human Geography.When my older children were in HS, the AP choices weren't allowed. However, I am 100% certain that at least my one dd could have easily handled an AP history in 9th grade (the other doesn't really enjoy history). The one who does went on to get 5s in APUSH, AP Euro, AP Comp. Pol and AP Gov. I am thrilled that my younger kids can take AP in 9th. Your dd will be competing with kids all over the country who CAN take these AP courses starting freshman year. Now, I probably sound like some kind of tiger mom, and I'm not - really. However, so much of what is done around different school districts seems to be simply to make it easier for them; it's not backed up by any proof (let me tell you how crazy I think it is that some schools won't let kids take Calc BC without a year of AB for prep - so glad that is not the case here). And as far as geography being important, I completely agree, but I don't necessarily think that you must take geography in 9th grade to achieve a good understanding of it. My dds' gifted program in middle school covered geography extensively, and through the other AP history courses, they covered much more. Personally, I think that the generalizations about US citizens not valuing geography (or math, or science) often don't apply to gifted kids.
I do agree that your letter is long (haha, this is definitely the pot calling the kettle black, since I'm totally guilty of this!), but I understand your frustration. I've often found that going higher up will help me find someone who sees the bigger picture. The gifted coordinator, Director of Curriculum, etc.. at the district, will often agree with me, and then the principal at the school is kind of stuck implementing it. To whom did you send this letter? Did they give you reasons in denying it or just say no?
Good luck
