Unfortunately, I don’t see a simple workaround. The reason why it works for your other example is that the metadata is really stored to the post type you’re on. So there is a one-on-one relationship between the post and the metadata. For your author’s use case, the relation is more complex. First, there is the relationship between the post and the author and then the relationship between the author and the related user meta. Although it will not be that difficult to create a possibility to sort on a simple value (field type is set to default), it will become much more complex when we also need to take the other field types into account. That is the reason why we did not (yet) create the support for that field.
The only workaround for you might be to create your own custom column with a custom implementation of the sorting model. The actual model is already available but it needs to be loaded from the custom column. If you like to go that road, please contact me at email@example.com so I can give you some tips.
About the upgrade. You should be able to upgrade your subscription from the subscription detail page on your account. There is an upgrade section with a button to upgrade it pro-rata.
Let me know if you have any further questions.