Why does Eren get the dream "See you later (Itterasshai)" in chapter 1 even before inheriting the Attack titan?
The reason Eren got the dream would be that Mikasa and Eren kissed and Ymir the Founder saw it (ch 138), so they synchronized. It’s unclear, but Mikasa may have kissed Eren in the same way in chapter 1, or there may have been something emotional about it.
Regarding the fact that there is no direct connection between the events in chapter 1 and Attack Titan’s power. Eren, the boy who woke up, doesn’t remember anything. So it’s not the same as seeing a memory. The “see you later” panel is merely a reader’s peek into his latent memories (those in Paths), and it is at the medal ceremony that Eren first actually recognizes them as memories. The process by which his memories are opened up is much the same as in the case of Historia (ch 54, 62, 63). He also subconsciously sensed that Mikasa’s hair had grown longer, but he did not compare the two visual images, dream and reality. This is what actually happened.