Ah, there should be a slider on the pricing page that lets you slide between plans. In hindsight, I can see how that might not be clear. Let me know if you still don't see it.
We do have some writers for whom English is a second language. We also have a handful of collaborators who have experience working with those folks. So far, those writers tend to be up front that English is their second language and so the contributors who jump into their work tend to self-select based on if they feel qualified to help. For now, this sort of emergent pairing of writers and contributors tends to give us more flexibility in what we can handle (as you noted, there's a lot of nuance within just this one use case).
And the tutor / tutorial model is certainly interesting (we've thought about it in a different context: hiring a writing coach), our general sense was that it is different enough from our core draft-level editing service that it's out of scope for the foreseeable future. I do agree there are certain things you can do live that are much harder (or impossible) to do asynchronously in doc comments.