Missing model error

Check the console of MCreator when the game renders it for the first time (in another words, as soon as you open the inventory and you see the broken texture). If you seen in the console something in red text about broken aspect ratio, is probabbly that the texture of the sword it's not 1:1

1:1 Aspect ratio means perfect square texture, for example:

32x32 Good
16x16 Good
8x16 Bad

And idk if it just me, but i tend to have problems updating .json models, it's like MCreator keeps in mind the "old" model if you import it and replace a model with the same name. Try to import the .json file with a different name, it has solved me some model issues in my current mod, but i'm using an old version of MCreator, so it might not be your case