A question for your question: When there's a duel between 2 players who only see each other's head, who is more skillful, the one who can accurately target the center of the enemy's head or the one whose crosshair just barely grazes the border?

I fail to understand how skill is involved in the scenario I did the test in; my crosshair is literally on the bots head, using an aimbot, absolutely zero human interaction involved, reducing all human error, so there is no "skill" involved. Not even all pro players combined could replicate the accuracy of an aimbot.

When there's a duel between 2 players who only see each other's head, who is more skillful, the one who accurately shoots at the enemy's head and misses, or the one who shoots next to the enemy's head and lands a headshot?

Knowing when you should and shouldn't take the shot and risks associated with it increase the skill ceiling;

This has absolutely nothing to do with the skill ceiling of aiming. Anyway, picture this scenario:

Bomb is planted on A site, you're in pit, enemy starts defusing, you have a clear shot and you can tell that he is defusing. You aim at the enemy's head, tap 5 bullets, miss all, and you lose the round. What did you do wrong here?

Inaccuracy reducing the skill ceiling is an entirely moot argument.

I said "as it reduces the skill ceiling of an important aspect of the game (aiming)", not the entire game's skill ceiling.