I think this is a question that can best be approached with a little help from logic definitions. I'm no philosopher, so please forgive this somewhat bumbling attempt.
In logic, the idea
if A, then B is called implication. But there are many ways of understanding this relationship. From a nice little introduction from lander.edu here are 4 ways to think about "If ... then ..." statements:
Logical: E. g., "If all philosophers are thinkers and John is a
philosopher, then John is a thinker."
Definitional: E. g., "If Carol is anemic, then Carol has a low
concentration of erythrocytes in her blood."
Causal: E. g., "If you strike the match, it will light."
Decisional: E. g., "If you donate to educational television, then
the company you work for will match the amount."
So the first example:
If he had not insulted me, [then] I wouldn't have slapped him.
is a causal implication. A caused B to happen.
Your final example:
If he had insulted me, [then] I wouldn't have slapped him
is a decisional implication. B is decided on when A happens.
The second "if ... then ..." statement:
If he had insulted you, [then] what would you have done?
is outside of any of these groups. It is an entirely hypothetical question, rather than a statement. It asks what would happen should A occur.
So with these definitions in mind, let's look at why the first conditional statement implies that the action already took place. Let's remove a few of the particulars, and look more generic cases.
If A had not happened, then I would not have done B - this is a statement of cause and effect. The hypothetical part of this statement (the "if" statement) is that A did not take place, which then implies that A did happen, and so B followed.
If A had happened, then I would not do B - the hypothetical part of this question is that A will occur. This statement then defines what would follow should A take place.