I got a 690 on the SAT II Latin, should I send this score, or will it hurt my admission chance?
690 on Latin will not hurt. Latin is Latin after all, very tough indeed and 690 is a solid score, there is some belief that crossing 700 is the magic number, but not true for all. If applying for engineering, then over 700 is important in math, etc. Even if you are looking at pursing med field, 690 in Latin is solid. I would send it. It is much better than sending in a high score/800 on a native language trying to present it as a second language, and representing it as an achievement 
Which for some funny reason, many students do.
I took the SAT three times and my scores were in the 1800s and 1900s but when I superscore it adds up to over 2000. Will sending in a low 1800 score affect my chances? Will USC look at all the scores individually or will they only focus on the superscore?