Each "Object" can only perform one action at a time.

Players can perform one action.

Monsters can perform one action.

Who these actions are performed on is irrelevant. If multiple monsters couldn't attack a player, then multiple players couldn't attack a monster.

Each action is run with respect to the Object that generated it. Each monster is able to perform their own action, one at a time, on any other Object.

Your player character does not react when guarding a monster's attack.

The monster attacks, and is met with your player's guard skill activation. That's their action, and that's how it's processed. Your character doesn't actually act at all.

I don't know how to make this any clearer for you, but it almost seems like you are intentionally failing to grasp the concept because it conflicts with what you want.

PS: Being able to throw some code into lolPython does not a programmer make. FFXI is and always will be constricted to the built-in limitations of the game's core combat engine. It was designed in such a way that no object, player or otherwise, could perform more than one action at a time - for very good reason from a development/troubleshooting standpoint.

This is not something that the Developers could change even if they wanted to. It is not something they could throw money or manpower at short of building a brand new game (hi FFXIV). That is the reality of this.