So when Dave and Rose flew to the Green Sun, did they hop over to the A2 session from there since the Green Sun is pretty much the center of all universes? If that's so, why did it take so long to cross the Yellow Mile and what exactly is that?

The Green Sun is in the Furthest Ring. The Furthest Ring is the space in between sessions. If you travel outside the ring of meteors around your session then you’ll be in the Furthest Ring. Sollux launched the meteor over to the Green Sun so the trolls could meet with Dave and Rose.

The yellow mile was the space between the two Fenestrated Panels, Jake’s and Jade’s. Jade explains why the trip takes so long here.