I think that swirl itself is the DBT. That swirl is where the 'data' for the DBT is 'stored'. The TD stands in the middle point between the DBT and the two worlds. Now, when you kill the TD, I think you in fact kill Schala, and not Lavos. Maybe the TD stayed in that middle point BECAUSE he had Schala, an undiscarded being, harnessed on him. Once Schala dies, nothing attactches him to the world, and back to the Tesseract we go.