If what I bolded is true(I have no idea, but it appears that you've researched this) then if it were my dd and it were up to me and not her doctor, I would have her get 2 shots.
I read the CDC info (synopsis of trial data, I think) and while these percentages are not exact due to me pulling them from memory, the gist of it was something like:
In children 10 and over, one single shot gave 70-something% immunity. It might have been 76%.... Or maybe it was 70% to some upper number %. Whatever.....The lower end of protection was in the 70s.
For children UNDER 10 who only received a single shot, vaccine effectiveness plummeted to 30-something percent. That's HUGE. I can't remember the percentage increase in effectiveness once they upped it to a two shot series, but I think it was similar to the effectiveness seen in the trials of 10 y.o. and up who were given a single shot. In other words, over 70%.
That's why it makes a BIG difference to me that DD9/about to turn 10 gets the correct number of shots. A scrawny 57 pounder who always gets the 6 y.o. dose of everything else.....I'd rather her be 70+% protected than 30+% protected. The CDC said the effectiveness had nothing to do with weight, like most medicine dosages, however. It was more that as children get older, their immune system changes and a single shot provides more protection that it did when they were young.
My hunch is, we'll go with 2 shots or 2 mists.