The cabin scene happens in the Paths same as the Armin meeting. It’s Eren and Mikasa “dreaming” of the life they weren’t able to have together. Similar to Eren and Armin visiting the far away places they had dreamed of when reading Armin’s book together.
The cabin scene would’ve happened in a timeline where Mikasa had confessed her feelings to Eren when they were in Marley together. When Mikasa calls him family. If Mikasa had confessed her feelings instead, Eren would’ve left everything behind to go and live in peace for the last four years Eren had left before his Titan timer ran out. Of course as a result, in that hypothetical timeline, Eren doesn’t go forward with the rumbling and he therefore gives up his ability to guarantee his friends’ safety.
The thing is, there is no alternate timeline in AOT. The future Eren saw when kissing Historia’s hand was the only timeline that would happen. Mikasa wouldn’t confess her feelings, Armin and Eren wouldn’t be able to look at the wonders of the world together, and Eren would always follow through with the Rumbling, dooming himself while saving his friends in the process.
Correct. That all happened in Paths. It’s all still “real” in that they do actually talk to each other before the end (all Eldians are connected to each other through Paths). It just didn’t happen in the physical world is all.