I have heard this before and still find it surprising, but am willing to accept it if it allows consistency with theories and calculations. But I dont find the demonstration totally convincing. Allan says there is no gravitational field inside the sphere because the force from around the sphere cancels out. But isnt this only true at the centre? Away from the centre isnt there a net force because such a point is closer to one side of the sphere? So if there is a field there before the collapse, as the field contracts it isnt the case that a field appears where there was no field before.Also what happens if the sphere collapses without generating power. The same region of supposedly no gravitational field will after collapse have the same field with out doing any work. All very interesting!

No explosion because the proper lengths of the pole and barn are such that the pole will not fit physically. The observed lengths are not the physical lengths and its the physical lengths that would be relevant to effecting an explosion event. This reminds me of Schrodingers cat!

George sees a steeper slope because the base of the triangle appears shorter to him.

We know we are not in an inertial frame because we know we are moving due to earth’s spinning and orbiting around sun. So our velocity is changing. But your argument doesn’t apply.There is no net force on us if we sit in a chair. If we sat on a chair on a non-spinning earth floating in space we would still feel pressure from the seat, yet would be in an inertial frame.