I think im a bit confused on the idea of someone doing everything MC has done? How far does it go? Is it all the same person? Did they go to the jewelery shop and got a blow job from val/fucked krystals mom

and on a more serious note what about Jenny? How does that work without having knowledge of the future/her location?
DID THE "YO MAMA GUY" AT THE POWERPLANT SAVE JENNY THE FIRST TIME?!?!
In this theory MC is either taking the place of someone (think of an NPC in an adventure/RPG game) if the event is important in some way, or the event simply didn't happen or happens differently if he did not travel back. The timeline is like the banks of a river; it tries to contain the flow of time and events, but if something comes along to disrupt the flow it tries to incorporate it into what should happen. If the perturbation is large enough then it has to adapt it as best it can. Think of a river where a beaver builds a dam. This may back up water until it can get diverted around it, maybe flooding a low-lying area that normally wouldn't be under water. Eventually the dam breaks and the area doesn't flood anymore and it's as it was before, or if it doesn't the river gets slightly wider at that point due to erosion and stays that way in perpetuity. MC going back in time is like a series of boulders that drop into the river at various places and the timeline has to make what he's doing work within the rest of the framework of time.
Let's take a couple examples with Khloe. MC stumbles across Jenny and Khloe in the bathroom stall. Jenny knows MC from the night visit and is attracted to him, and Khloe thinks he's hot plus she is trying to get Jenny to explore a little bit. In the original timeline it could be some other lucky dude who finds them and gets the benefit of the activities, or its some other guy who neither knows and isn't that good looking so nothing happens. In the jewelry shop the original timeline has (let me make up a character) Brian, the captain of the school tennis team who Khloe has hooked up with a couple of times because he's nice, good looking, athletic, etc. and now convinces him to help them both out. It's important that this occurs since that's how Valerie starts becoming more sexually adventurous, which is necessary to convince Tiffany to get involved sexually with MC, all part of the core timeline.
Another example would be that the shit ninja saves Darci's mom solo, but with MC and Darci on the hunt for her they wind up doing much of the saving instead. The end result is the same but they get incorporated into the actual events.
One corollary to the "Jenny is Jackie" theory is that having time travelled that perhaps future Jackie remembers the original non-MC timeline (she and Tiffany dance, talk, etc. all night and go to bed but she's not woken up by MC) as well as the version where she comes after MC with the baseball bat. She knows something is going on, and MC plays a part in it but doesn't know what the outcome is so doesn't provide him any information that might change what he has to do naturally in the various situations he finds himself in.