from Guns, Germs, and Steel, how does Diamond's theory that invention is, in fact, the mother of necessity bear upon the traditional "heroic" model...

This is an interesting
concept.

The heroic model supposes that any invention is a
product created by creative and unique individuals.  Sometimes this will be known as the
'great man' hypothesis.

Diamond's theory challenges this by
showing that different civilizations and societies throughout history have made similar
discoveries - simultaneously without any contact with the other groups.  Good examples
might include the cultivation of crops and animal husbandry.  Going further, we have
pottery, architecture and later writing.

Does this mean
every civilization would have come up with the lightbulb eventually?  Good question. 
The modern inventions as we know them were all products of a particular civilization. 
Diamond would argue that steel, guns etc. were creations from Western civilization
thanks to its fortunate advantages in the past.  There is therefore no way of
scientifically proving this theory.

I will let you come up
with your own ideas and conclusions from these initial ideas.  Hope that
helps