Ok. It's fine.
My own quest is done.
Eccma's quest has started.
—
At the very least, I've already figured out a way for modders to add items to traders without overwriting dialogs.
While Eccma's quest is not yet done all of this will not happen centrally. But it will still work.
Eccma can use json method for simple cases (may be it's possible to add some tags into items in jsons of traders, for story_stats[""] or actor.stat[""] check? ShopNerf is already added by me in code above), and use old method when it's something really unique, and slowly extend the json method. Let's say, Eccma can use a hybrid way, for short. And even that is enough for modders to connect to the existing structure and use it then.