It sounds like you are asking a bunch of different questions. I have two 7th graders and I know whether and when they are challenged because I talk to them all the time plus I will look at some of their essays, reports and projects even though I have rarely helped with school work since mid-elementary school. I would definitely not base judgment on their grade percentages or whether they choose to study. In other words, a 105% is not necessarily an indication of superior ability to a 95% nor is choosing not to study an indication of a higher intellect that choosing to study.
As for self-study projects, I think that could be a key way to assess whether a student is really "above" the rest as well as provide an opportunity to stretch their intellectual muscles. Of course, I am not familiar with the dynamics at your DD's school, but I don't see how undertaking a self-study project would isolate her more socially, particularly when some of the other students are doing self-study projects.
I agree that the Explore scores would not be particularly persuasive for 6th graders even if they hit the ceiling since there is not much room by that point. Talent searches usually test in January or February and the norms are for Fall 8th grade so at most you can claim 1 1/2 years "ahead". By the end of 6th grade, it makes more sense to test on the ACT or the SAT if you are trying to prove that your DDs need higher level work.
Our district also has MAP and PARCC for national comparisons as well as district benchmarks which are more curriculum based. Perhaps check to see the actual substance of their specific assessments to figure out whether they can be of any use.
Having said all that, I am not sure what providing "proof" will do unless you are trying to grade skip or subject accelerate. We are in a decent school within a decent district so perhaps your situation is outside my comprehension. We have found that 7th grade has been a substantial leap in academic demands and independence from 6th grade when they were partly focused on acclimating the kids to the executive functions and social demands of middle school. However, even in 6th grade, I noticed how open ended some of the assignments/projects were so that it really was up to my kids to stretch themselves if they chose to do so. Furthermore, there were some in-depth and sophisticated classroom discussions in which my kids participated. That's not to say that a significant chunk of school assignments weren't basic or that a goodly portion of their "gifted" classmates weren't left in the dust.
I can't say that middle school is an academic fit for either of my kids but there certainly are much more opportunities to shape a more tolerable fit than in elementary school. More importantly, I have been impressed with the growth in social and leadership skills, in large part due to group projects and presentations.
