Rox and Buddy are talking about Scorsese's movie "The departed", so that scene is around 2006. 13 years from 2019, the year BaDIK is set, I suppose (It's the year of the first release). If Quinn is 19-20 now (she was a "daughter" last year), she could be 6-7 then and Jill around 8. Makes sense. Lana could be 18. She couldn't be underaged, because I don't think a father could make an underaged girl leave he familiar home. I don't see a big problem here with ages.
Anyway, I think that if DPC wanted to give a hint that this girl was Lana, she would have used nail polish. She always uses nail polish in the others images of her. Probably it's just a random butt. But that's not important. The theory can be true and it's not needed that Lana is that girl. She could have a drug-addiction problem that at the end caused her death, and be involved with Rox and Buddy. It would be a way to link Jill's and Quinn's stories, even if MC is not in Quinn's path. We'll see, I suppose.