Vs - velocity on beginning
ve - velocity on ending. You've got:

So he needed 4 second.
All of the observations except "powerful gravitational field" are consistent with the current theory of black holes.
The gavitational field of a black hole is thought to be no different than that of an ordinary star with the same mass.
(2) impulse is a vector quantity. Energy has no direction, nor does power. Work is a dot product of vector quantities, which makes it a scalar.