1) I don't remember having to mark anything "not applicable". I just zeroed out the original attempt and the final attempt was marked as repeated and entered in the credits.
Edit: Make SURE you zero out the original attempt because if you don't AACOMAS won't be able to fix it for you. AACOMAS originally counted my first attempts for two repeated courses into my GPA even though I zeroed them out, but after a LOT of communication with them, they realized they made the error and re-verified my app and updated my GPAs, which made a HUGE difference in my GPAs. Really go over this and review your verified GPAs to make sure they didn't make a mistake like they did with mine.
2) I think you can just mark YES for completed and then you'll have to send your score to each school or however it works. Schools only look at official score reports anyway so it's not a big deal to not enter an unofficial score on the AACOMAS app.
3) I took courses at both semester and quarter schools and did not have to convert credits. So just enter the credits as they appear on your transcript and you'll be fine.
4) Can't answer this really but I recommend using Interfolio and saving your LORs there in case you need to reapply. If you choose to use Interfolio, you will need LOR writers email addresses and Interfolio has directions on how to send to AACOMAS.
5) I made an account last year as well just to see what my GPAs would be and I think I filled out some of the app. I had it carried over for this cycle, but we will not be seen as reapplicants since your app was not verified previously.
6) If your major isn't listed and there isn't an option for "other" and you type it in, I would just go with something that is most similar. Your true major will be on your transcripts for adcoms to see so I don't think it will be a huge deal.
7) I would believe a committee letter would be included in the 6, but I am not positive. Why so many LORs? Some schools only want 2!
8) I took a course that did not have a designated subject on AACOMAS. I emailed someone in the course's department at my UG and asked for a recommendation on what to list it as. You should do this first and see what they say.
Hope this helps!