Oh my god, 4-8. Even though you mentioned it, I had no idea what you were trying to do, especially when you were messing around to get through the switch doors without using a bomb. And yeah, the border blocks created when no level exists above/below are still indestructible. If they weren't, though, I think you very well could've walked on top of the levels all the way to Area 6, then used a bomb to blow your way into 6-9 (or just fall into 6-1).
Also, I should fix the intermissions showing "0.0" clear times, huh.