If I was lying I couldn't prove what I said, unfortunately for you I can. Zoro said if Luffy accepts Usopp into the crew again after Usopp leaving and without Usopp apologizing from Luffy, Zoro said he would leave the crew. So as you can see nothing stops Zoro from leaving the crew. This ''he gave his word he can't dishonor himself'' thing of yours got debunked very hard because surely Zoro couldn't threaten Luffy with leaving the crew because he gave his word according to your fanfiction, there's no such a thing like that as Zoro told Luffy that he could leave the crew easily if Luffy accepted Usopp without Usopp apologizing from Luffy.
Zoro can leave the crew whenever he wants as he can even threaten Luffy with leaving it.
That's 1.
And the 2 is; he literally said Luffy can't be his captain without being stronger than himself in the fishman island, Zoro himself said Luffy being the captain is related to Luffy's superior power when Luffy one-shotted 50.000 men. That's also Zoro, so where was his imaginary word when he said that his captain must be stronger otherwise Zoro becomes the captain? There's no such a thing, so its better to stop crying about this.