I suspect the main reason the spec doesn’t yet define conventions for partial success is that we don’t yet support bulk operations. I.e., a request is always supposed to operate on only one resource at a time, and for operations on single resources, it’s simpler if the operation must fail or succeed completely. When we standardize the extension system + the bulk extension, support for things like this may get better. But, until then, I think you should try to restructure your resources so that one response code is sufficient. Again, if you post more about your particular data model, we can help further.
Cheers,
Ethan