I think Mikasaās dream was kinda like Arminās where Eren brought her to the Paths and made the setting to where they were able to be together as a way to talk to her. I'm guessing he did this to everybody else considering how they found out his true reason for doing all this.
The Ymir and Mikasa thing was not explained which also frustrated me but I was thinking maybe it was about how Ymir saw herself in Mikasa. Ymir loved King Fritz to death and Mikasa would even give up her life for Eren so maybe that's what they meant. I guess it couldn't be explained since Isayama made it that even Eren didn't know the reason. I just thought this would be the most plausible reason to ease myself from further headaches from overthinking haha
Yeah maybe it's the same as what happened with Armin. But it still seemed a little bit different to me, since there is the "waking up from a long dream" that she says, and eren says the same thing in the begining of the manga. To me something is missing, I guess Isayama wanted to leave it like that, for people to interpret it the way they want.
I also saw the similarities between Ymir and Mikasa, even if it's not really the same situation. But I wonder, if Ymir is able to see everyone's life and feelings, maybe she saw mikasa's devotion to eren and decided that she wants to see her kill eren to be free (because for her, her love made her unable to change her status of a slave) . But we also see that Mikasa chose to not forget Eren, she's free but her feelings are still there even after Eren's death. Anyway it was a good read, but it's too sad and depressing.
Do you think Eren could've talked to his past self and made his younger self forget like what he did to Armin? Because if he can, I'm guessing that the part where Eren says he woke up from a long dream in the beginning of the story was when older Eren talked to his younger past self and showed the future events, like the flashing of pictures Eren saw in the beginning of the story. Maybe I'm thinking too much like I always do but this was the conclusion I came up with
I think I'm going to change my interpretation of the Ymir and Mikasa thing a bit. So Ymir loved King Fritz so much that she did everything for him. Even after her death she still did what King Fritz wanted. Mikasa, on the other hand, was someone she saw herself in. Mikasa loved Eren so much, she was so loyal but in the end, she was the one who killed the person she loved the most. I guess she was, for lack of a better word, comforted knowing that someone was willing to give up their love and kill that person.

I'm crying for Mikasa, what kind of life is that, she lost all the people she loved, and had to kill the guy she loved the most. They could have at least make her move on, or be with the others. Also, it's disappointing that the Ymir Link with Mikasa was never explained, as well as mikasa's and eren's dream.