1) It's still a gray issue, and depending on the situation, the
right for self-defence is called for when your life is in danger,
thus it will depend on the context that you are referring to, and
it goes for 2) as well.
As for (4), both of you might be charged but then again this
depends on the situation.. On (3), doubt so, no one is given the
the right to harm any other if no danger precedes..
And (5) heh, if that's the case the number of arrest cases will
surely rocket skyhigh since a true martial arts exponent would be
able to wield his body alone as a weapon..
Just my 2 cents 