...I just had a few questions:
1. Will my low essay score affect me?
Firstly it’s important to remember that not all colleges requires the SAT essay score. Some colleges require that you apply with the SAT with Essay; others don’t care whether you submit an SAT score with or without the essay. Many don’t require it because they feel that the application demonstrates an applicant’s writing skills sufficiently. According to a Kaplan poll in which 300 colleges were were surveyed, most schools will not require the optional SAT essay. However, some still do recommend or require it, particularly in the most selective tier of institutions. https://www.bloomberg.com/news/articles/2015-08-24/it-s-yale-vs-brown-in-ivy-league-split-over-optional-sat-essay
I note that you are interested in applying to the UCs and University of Michigan. All of these universities to require that you submit the essay score. You can find a complete list at this link.
In relation to the essay scores importance, the general consensus is that the essay is the least important part of the SAT overall. Admissions offices will look much more closely at your composite score.
However scoring poorly on the essay could be a red flag to admissions officers that you might not be fully prepared for college-level work. Let say you say got a 9/24 on the essay, then there would be more reason for concern. In your case your essay score is more or less consistent with your other strong test scores so I wouldn’t stress too much.
2. I got an 800 in math even though I got 1 question wrong in that section, so is there a curve? If so, is the curve based on other Nov 2016 SAT scores or is it a standard curve based on averages?
Yes and no. There is a curve but not based on the performance of those who sat the test at the same time. The curve is based on the difficulty of the exam that was given on that day. In other words your score takes into account the difficulty of the SAT test the day you sat it.
The reason you score is a perfect score compared with the raw score is because any questions you answer incorrectly are not counted. https://collegereadiness.collegeboard.org/sat/inside-the-test/compare-old-new-specifications
3. What does the percentile number mean - am I being compared to other Nov 2016 SAT scores or just the "average"?
There should be two percentile ranks on your SAT score card: the Nationally Representative Sample Percentile rank and the SAT User Percentile – National Percentiles. The Nationally Representative Sample Percentile rank is based on research study samples of U.S. students in the 11th and 12th grade weighted to represent all U.S. regardless of whether they have taken the SAT or not. The SAT user percentile is derived via a research study sample of U.S. college bound students in the 11th and 12th grades who have taken the SAT last as 11th- or 12th-graders. For example, a student’s score in the 75th percentile means that 75 percent of the user group of U.S. students last testing as 11th- or 12th-graders would have had scores at or below that student’s score. (https://collegereadiness.collegeboard.org/pdf/understanding-sat-scores-2016.pdf)