I would also encourage you to cast a wider net. While the Harness method is great, it’s not exclusive to Exeter and many other schools have engaged students and discussion based learning as well. Most of these schools have Finance Clubs and also offer independent study in a number of topics (if that’s what you’re seeking). Are you that far advanced beyond the HS curriculum in any subject? The reasons that you’ve outlined aren’t exclusive to Exeter.
Your 7th Grade grades are going to be an issue, especially since you don’t have a full term of 8th Grade grades yet. I think that you should proactively bring this up in your interview so that you can offer some context–not excuses. It should be clear how you’ve overcome this hurdle and changed. I agree with other posters that making excuses will make admission more difficult.
Exeter AOs value independence and a blip in 7th grade may indicate that you need a smaller school that’s more nurturing or that you should wait another year until you have a strong 8th Grade record to show how you’ve changed. This would help to dispel any concern from AOs that you’re a capable student (95% SSAT) who lacks motivation and won’t thrive at Exeter.