TL;DR
Pawn or Source Pawn? Which are you actually asking about. None of the code you posted is Source Pawn.
EDIT: Ok, I skimmed over it:
#1: Example:
amxmodx.inc
PHP Code:
#if defined _amxmodx_included
#endinput
#endif
#define _amxmodx_included
#3: Why would it be checked 10 times?????? If it's called once, it gets called once.
#4: A switch() is always more efficient than an if else if structure.
#3&4: If you want to know more then you should first read the AMX Mod X wiki. There are some explanations of why some things are more efficient than others.
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