There seems to be some confusion here as to when/where what happened over the course of the chapter… and I guess I can't blame people for being confused. Its structure seems to be a main flashback to the days 50 years ago with Laboon while Brook plays the piano, along with a flashback within the 47-years-ago flashback.
Basically, Brook plays "that song" the piano and thinks back to how (starting off the morning with "that song") they found Laboon, how he followed them to the twin capes, and how they left him behind with Crocus. Back in the present, Brook cries over Laboon.
Then, he thinks back on how, 3 years later, he came back to life on a ship in the Florian Triangle. Finding his crewmembers all dead, he placed their skulls in a coffin, then noticed that he himself was now no more than a skeleton. In his time alone there, he reflected back to when the captain fell ill and decided to leave the Grand Line on their original ship, which appears to be prior to their entering the Triangle. The chapter then comes full circle, as Brook, undead and alone on his wrecked ship, begins singing "that song".
The theme that connects the first flashback, the second flashback, and the flashback within the second flashback to the present day of the series, is the song about Binks' booze.