I know they purposely left off some heat tiles to test that eventuality - was the flap where they did that? I guess so, given the number of cameras pointing at it.
No, those were at the unpressurized section by the engines. You can see from when the plasma originally came though it was the gap for the hinge. There needs to be a gap there for them to move, so I guess the plasma found it's way though the gap.
Wow, that's even more impressive then - they added a defect and a different problem occurred and it still landed. I caught some of the stream and heard the SpaceX audience's reaction to that fin burning away - I thought they sounded more excited than worried.