Could anything have been gained by including more scenes from the time before or after the events of Animal Farm's plot?

I think that it depends on whom Orwell thought of as his
audience.  For example, at the time the book was published, Orwell obviously didn't care
if he offended England's "Allies" or other readers--he simply wanted to satirize the
many problems of the Soviet Union's totalitarian regime and to demonstrate how a
"people's" revolution can go awry.  During Orwell's brief lifetime, I don't think that
he needed to provide any further events because his readers should have already been
familiar with the failure of the Bolshevik Revolution in Russia to bring about a
prosperous, free state.

That being said, if Orwell intended
for Animal Farm to be timeless, if he had added more events before
the book's opening, readers probably would not need as much background knowledge to
truly appreciate the book.  As far as adding more events to the conclusion of the book,
that might be problematic because Orwell did not live long enough to see the downfall of
the Soviet Union.  Similarly, in Animal Farm and
1984, Orwell seeks to illustrate the hopelessness of the common man
who lives in a totalitarian state.  Thus, the animals plod on at the end of the book
while the pigs gain more power, and nothing improves.