why did he scrape her back?

1)If you are Vietnamese, you would know that scraping somebody's back with some ointment is a sort of treatment among Vietnamese when you are sick. The Vietnamese believe it helps to bring down a fever, for instance. It's hardly pleasant but obviously in this case, the 2 characters seemed to feel pretty erotic about it. It's symbolic in this film also because the prostitute didn't look for customers ( she's sick ) anymore & it's some sort of healing process for her.
2) You guessed it right. The traditional Vietnamese white dress is a uniform for girls in public high schools in the South Viet-Nam. And they wore it proudly ( I don't know if it still exists after the Communists invaded the South in 1975 ). Vienamese women after high school, in Viet-Nam or abroad, would think twice before putting on something like that because it symbolizes so many things : youth, innocence, untainted, educated,etc. ... Also the red flowers from the mohur trees always associated with Vietnamese high school girls ( more than with boys ).