I think I'm not alone when I say that the new USM roll-out has been nothing short of a nightmare. aside from the technical issues plaguing the Admin side of things, which I know you are working on, the model itself is simply ridiculous. It is difficult to manage and takes control over who gets a license and who doesn't away from the business. It also creates a lot of unnecessary work to manage the provisional members. For a platform that exists to help create efficiencies, it is ironic you went with such an inefficient and badly designed user subscription model.
As you work toward addressing the issues that many of us are experiencing, I would like to suggest an alternative. Namely, that you divide users into three categories. Creators, Editors and Viewers. The Creators are the current "members." Editors are the former free collaborators, but no longer free. Viewer would continue to be the only "free" option. The Editor level would have a lower price point than the Creators. The "creating a membership by sharing" would go away entirely, and any billable change to a membership would have to be granted by a System Administrator.
I believe this would be a better compromise between your needs and the needs of the customer.