David wrote:

With a vertical monopole antenna, the field emitted by the radials forms a
near field and interacts with the wave radiated by the vertical element. Is
it mainly in the vertical direction that the radial fields interact with the
field from vertical element?

The fields "interact" wherever they exist.

I would expect the antenna feedpoint impedance
to be formed from the length of antenna conductors and the interaction of
the fields.

You should also expect the feedpoint impedance to be formed by the
ratio of the voltage to the current at the feedpoint, among other things.

If a monopole is simulated in a NEC program above a perfect ground plane, is
it possible to see the reflection?

Only on Groundhog Day. :-)

I would expect NEC program to show
reflection of waves from metallic surfaces e.g reflection from perfect
ground plane or parabolic dish. Can a NEC program allow the user to see
whether the wave is reflected (as for perfect ground plane) or whether
effect is due to wave interaction (as for radials)? Can the user then vary a
finite size ground plane to see whether antenna impedance comes from
reflection or wave interaction?

An interactive video game it's not. Perhaps if you were to become a
NEC user, you would come to know these things and more, first hand.

Jim, AC6XG