I would not be overly concerned with how
Keystone administers its exams. As long as it is properly accredited, there should be no problem. And Keystone is most certainly properly accredited.
As for the
University of Missouri High School and
Indiana University High School, these are both very good schools. Importantly, they will be viewed as better schools than Keystone because of their relationship to universities. This perceived status may help you get into a better university when the time comes.
I would talk with both schools to see who an acceptable proctor might be. Often a teacher, librarian, or professor is acceptable (but I do not know the specific rules for these schools).
Tom Nixon