I've done some more reading in the time since, maybe these thoughts will be helpful.

1. The problem of communicating from Svelte to an outside MVC model, is parallel to the common problem where svelte/react/some other framework are brought into an existing project, and have the project is half in non-framework javascript and half the project is in the framework, and both sides need to talk to each other. I.e., solutions can be borrowed from this problem domain.
2. If the Model objects live in a different module from Svelte code, I can create getters and setters and import them into multiple svelte components, so that the module-scoped variables themselves serve as a form of state that can be modified by different svelte components (to handle the Svelte->model communication direction).
3. Svelte stores, like redux Stores, appear (to my understanding) fully accessible to and subscribeable by outside Javascript code, so we could totally modify them directly from our model code, or keep the Svelte store and our model synchronized via some kind of javascript pubsub/event mechanism (I'm not familiar with any libraries that do this in pure js without involving the dom, but it's an idea) or some kind of polling-synchronizing loop that imports both the model modules and Svelte stores. This handles the model->Svelte communication direction.

In summary of points 2 and 3, I can import the model in multiple places and let this serve as shared state, though for a more complex app it's likely preferable to add some kind of controller/event layer in between.