But, according to the letters, obtained by TMZ Sports, at one point later on during the evening ... he "violently" sexually assaulted a woman inside of a men's bathroom.
In the letters, Mitchell claims that NBA and Heat security helped separate the woman from her friend ... and then forced her into a restroom that McGregor and his security guard were already inside of.
In the letters, Mitchell says "security refused to let [the woman] exit or allow anyone else, including her friend, inside the bathroom." The lawyer then claims McGregor emerged from inside of a handicap stall "and shoved his tongue in the victim's mouth and aggressively kissed her."
According to the letters, the woman was able to get McGregor off of her by telling him she had to urinate ... but then McGregor allegedly forced her to have oral sex with him.
McGregor, 34, is then accused of grabbing the woman and pinning her against the wall ... before attempting to sodomize her.