Your points are really interesting. There are many things that I confess I would never have thought of before, and some I was thinking about too much, while I was theorizing here with some friends about the possible continuation of the story.
Regarding Rebecca being her canon girlfriend, given that we now have her Christmas special as a sort of "ending" to the story, that's the only way to understand it. But if the dev hadn't abandoned the game, I feel like every year he'd make a Christmas-themed "ending" for each main girl. I think the Rebecca special came about because the community and fans of the game at the time were asking a lot for sexual content with Rebecca, since most of them really like her.
But realistically speaking, in my opinion, at first I understood that the main girls were Jenny and Ashley, who were sort of the "leaders of their gang," so to speak. But as time went on, the dev created a very detailed (and melancholic) backstory for each girl in Jenny's gang, and later for the girls in Ashley's gang. So I think the dev would have ideas for routes focused on each of the girls later on, or in a sort of "harem" (given by the last images we saw, for example, in Jenny's house).
Now, let's move on to the events of Ep. 6 that I thought would happen.
- I believed that the sneak peeks of the girls in lingerie in front of the mirror would be if you followed a specific girl's route, and that there would be a moment where we would need to buy clothes to wear at the party venue, where the MC and the chosen girl are going. But I feel like you would have the option of going clothes shopping with up to two girls (Jenny's entire gang regardless of your choice will buy the clothes, since they were all invited to the party), since I remember being able to date both Jenny and Vicky at the same time, for example. Then, depending on whether you chose to go with the girl from Ashley's gang, you would go to the swimming pool with her and have a romantic/sexual moment. If you chose to go with a girl from Jenny's gang, you would have a moment alone with her at Jenny's house later, and soon after, I believe we would see the rest of Jenny's gang wanting to show off the nightgowns they bought at the store and then play with the MC (which explains the photo of the girls in nightgowns and then topless in front of the MC).
Now about the part where things go really bad for Adam, this was the most interesting part to theorize with my friends, because either way, I'm sure Adam would end up upsetting the MC by trying to humiliate him, but depending on the choices (which, to be honest, weren't many) and the routes of the chosen girl, we could see a possible outcome from this situation:
A) Either someone from the cartel, who was present without our knowledge, would end up ruining the party and giving Adam a big scare (or kidnapping him... or worse), and then Mr. White would help the MC and Jenny's gang leave the city and go to a house on the coast (just like the one in the MC's strange dreams);
B) Or Mr. White himself would end up ruining the whole party by trying to turn things around, letting everyone in the room know how despicable Adam is, revealing details of the blackmail he had committed and other evidence he possibly had there to help Jenny and the others, and incriminating Adam right away. But that way, I'm not sure if the Cartel gang would show up later and continue from there...;
C) The MC himself, with a gun stashed there, would react to yet another attempt at humiliation from Adam, which, depending on the situation, could be Adam's last attempt at humiliation, leaving you to decide Adam's fate right there in his hands. But that way, things might not go well for the MC depending on his choice, but he could leave alone, or the girls would leave without him as a result.
These were my ideas about it, the other things that I didn't answer specifically, it was because I share the same idea that you told me, and I totally agree!