first off you always get at least one dice for defense or a skill unless it says you can not default. And yes to the second one.
The rules never state this anywhere I'm aware of. Cite it please.
Actual Rules to the contrary....
p61-62 SR4a "Long Shots"
"In some circumstances, modifiers may reduce a character's dice pool to 0 or below. In this case, the character automatically FAILS the test...." spend edge long shot test (which answers Erich's second question). Nowhere in the rules am I aware of anything which says a character always gets at least 1 die on any test.
So if your reaction pool is reduced to zero... you can spend 1 point of edge... and roll your edge and only your edge to go for broke on that one defense test.
(oh dear. the fun people would have with MrLucky type stuff then... yeah I role 1 die with my pet rented 'guard' spirit to never glitch... 33% of the time... miracles happen, and when I really care I actually spend edge).
Tecumseh:
No, each shot is most definitively a separate attack.
The first shot is resolved fully... attack, hit, damage soak... before the second is declared.