I think this interface is less interesting than the semantics behind it, particularly when it comes to concurrency: what happens when you delete a folder, and then try and create a file in that folder at the same time? What happens when you move a folder to a new location, and during that move, delete the new or old folders?

Like yes, for your mum's use case, with a single user, it's probably not all that important that you cover those edge cases, but every time I've built pseudo-filesystems on top of non-filesystem storage APIs, those sorts of semantic questions have been where all the problems have hidden. It's not particularly hard to implement the interface you've described, but it's very hard to do it in such a way that, for example, you never have dangling files that exist but aren't contained in any folder, or that you never have multiple files with the same path, and so on.