Answer
It's been awhile since you posted this ID request. Not sure if you ever got an answer, but the movie is Clover's Night vol 19. I don't know about the original movie studio but it seems to be distributed in the west by Suave Communications.
I think the girl in it is Kozue Matsuyama.
Not sure about the mistress.
Just google the movie title and you'll find a goodly amount of links.
I agree it's a hot scene.