Hey - so here is my two cents, none of it is authoritative though.
1.) I don't think you HAVE to explain why you want to move back to the US, but it may be helpful?
2.) I don't think you need to provide an explanation of the grading system, the report compiled by LSAC is probably the only legitimate way of doing this (as it places all internationals through the same process making it vaguely fair - debatable). Have you submitted transcripts yet?
3.) LSAT...yes, it seems to be weighted heavily from what we can tell. I personally have a somewhat different opinion to a lot of people, with regards to LSAT score and where you apply. I fully understand the percentiles, but I wonder if people sometimes forget that 25% of students are UNDER that level...sometimes that's 100 students in a school. Obviously this would be more of a reach than a target school, but I think it's still worth giving it a shot - particularly if there are some other aspects of your app that are outstanding.
Hope that helps, and welcome to the thread