Number one is to trust what @Prish said. Basically, to trust yourself and your own instincts.
But I think those (good) instincts are why you asked this question…
It was a big breakthrough for me to realize that other characters can contribute to or exist “in” the MC throughline. You can do this as much as you want. The key thing is that it’s the MC himself who has the “I” perspective and is experiencing the personal issues and conflict. But the other characters can make that conflict apparent, make it happen, make things tough for the MC.
A lot of the time these other characters will also exist in the OS, and it’s possible some of their interactions with the MC may do double-duty. But sometimes they may just serve the MC throughline. (I think @MWollaeger suggested at one point that Andie’s friends in The Devil Wears Prada were almost entirely MC throughline characters.)
Here’s a funny example from my own writing:
- In my current story, my MC’s two best friends and his parents all contribute to his throughline.
- Back in the beginning of Act 2, I had identified my MC’s friend Eric as being the Skeptic character in the OS. I thought that was kind of cool as I had never tried to make that happen, yet he was clearly Opposing and Disbelieving left and right especially with anything related to the Goal.
- Then suddenly I had this scene where Eric was being all supportive of the MC. For a while I thought I was doing something wrong, but it really felt right for him to be supportive at this point. (“Dude, you’ve never had a better chance to get with this girl!* Come on, I’ll drive you.”) Then I realized his Support was entirely in the MC throughline! In fact, later on when the OS stuff starts to intrude again, his support evaporates and he gets all Oppose/Disbelief again. “This chick is involved in what?! No, she can’t borrow my truck!”
- Elsewhere in the story, Eric helps illustrate the MC Symptom of Logic with his attempts to get Devin (MC) to follow this proscribed, logical way of picking up girls, which Devin can’t stand.
* His actual words were a lot more crass
Now, your specific questions and example sound a bit complex for me:
Honestly the Element-level stuff I would mostly just leave up to the subconscious and do by feel, at least on the first draft. Rather than thinking about who’s using what elements, think about pushing the MC on his personal issues.
Same with Story Weaving. Just do what comes naturally. I find it helps to know that any particular “thing” in your story (event, beat, action, line of dialog, entire scene, whatever) can serve any number of throughlines.