My own interpretation is actually different from any of the ones you've listed.
We discover over the last few episodes of Season 2 that Yukinoshita has a lot of trouble even identifying what it is that she personally wants. Where Hikigaya has to have an external reason to help people but is otherwise incredibly self-sufficient, Yukinoshita needs an external reason to do practically anything at all (mainly via her mother and sister), and once she has an external reason she forces herself to conform to it. Yuigahama may not be intellectually bright, but she's incredibly perceptive when it comes to people's emotions and social cues, and she has picked up on this fact. She is also perfectly aware that Yukinoshita is starting to feel romantically inclined to Hikigaya, which puts Yuigahama in a very difficult place: she wants to maintain her friendship with Yukinoshita, but the entire reason she befriended her in the first place was to get close to Hikigaya in a way that won't cause her to be judged or ostracized by her social circle.
Additionally, Yuigahama is aware that Hikigaya is undergoing an emotional shift of his own and is allowing himself to become closer to both her and Yukinoshita. This is simultaneously thrilling (because it means that she has successfully gotten him to notice her, finally) and terrifying (because she recognizes that she's in a love triangle, and him choosing one of the girls is going to destroy their current social relationships no matter what).
So the question becomes: how can Yuigahama gain what she most wants (a romantic relationship with Hikigaya) without destroying their current social dynamic? Well, first off she can't come out and explicitly confess to Hikigaya. No matter how he responds to that, their social dynamic is dead (this is the same problem that came up with the Ebina/Tobe interaction earlier in the season). Additionally, she is feeling guilty about taking advantage of Yukinoshita's obvious lack of experience when it comes to dealing with romantic feelings.
The answer that she arrives at is her double date (cherishing and celebrating their current social dynamic) and final confrontation with Yukinoshita (which may destroy their social dynamic).
The trouble is that because Yuigahama cannot explicitly say what she wants to say, we have to rely largely on subtext. As I read it, the subtext goes like this:
- She gives the cookies to Hikigaya. This is a clear indicator to both Hikigaya and Yukinoshita that she is interested in Hikigaya; she is tacitly admitting that he was the one she wanted to make cookies for all along.
- She then tells Yukinoshita that she "wants it all". My reading of this is she is saying, "I want Hikigaya, but I also want to maintain my friendship with you." Of course, there is only one way this can work, which is if Yukinoshita gives up on her own feelings for Hikigaya.
- She then assures Yukinoshita that if Yukinoshita leaves everything to her, she will ensure that outcome. This little bit of manipulation is pretty brilliant (and underhanded) because it plays right into Yukinoshita's inability to identify and act on her own desires. Yuigahama is saying, "I know it's difficult for you to act on your desires, so why not act on my desires instead? I will give you the friendship that you've never been able to gain on your own as long as you give up your desire for Hikigaya."
And at this point, Yuigahama has won the love triangle match. Until, that is, Hikigaya interjects with his insistence that Yukinoshita needs to solve her problems on her own or she will never escape the cycle she's currently stuck in.
At which point the really fascinating part of this scene occurs, which is that Yuigahama says she thought he would respond that way and immediately backs down. The implication here is that Yuigahama entered into this whole gambit believing that she would probably fail, but did it anyway because she did not think the status quo was maintainable and that forcing her friend and her crush to face the emotional currents that underly their little threesome was desirable even if it lead to her losing Hikigaya.
I really like Yui Yuigahama.